Proc. Soc. Hort. Sci.
7: 33-46 (April, 1911)
THE REST PERIOD IN PLANTS
W. L. HOWARD
University of Missouri, Columbia, Missouri.
The rest period of plants is the phenomenon which we know as dormancy. Since plants become dormant in summer as well as winter, we will, for the sake of convenience, recognize two general kinds of rest. Those belonging to the winter rest class are the outdoor growing forms of the temperate zone, including the woody plants both deciduous and evergreen, and the hardy herbaceous biennials and perennials. The summer rest for the greater part is confined to bulbous plants which may or may not be hardy enough to withstand outdoor conditions all the year round.
Besides the winter and summer rest we might recognize a third form which seems to be independent of season, but which is pretty apt to conform with the winter rest. This form of rest is confined to seeds.
The rest or dormant period in plants varies greatly in the different species and varieties. Latitude, climate, species and perhaps heredity are influencing factors in determining the time of its occurrence, length of duration and degree of intensity.
The rest period is especially noticeable in deciduous woody plants growing in the temperate zone because of their annual leaf-fall in autumn and resumption of growth again in spring. In seeds and bulbs it is not so apparent to the casual observer chiefly because they are not seen so often or displayed so prominently.
In woody plants growing out of doors, and sometimes indoors, too, the rest period begins in late summer or fall in most cases, in the temperate zone, and continues until late winter or early spring. Just when the rest begins is difficult to say. In woody plants, after the formation of the terminal bud, length of growth, of course, ceases or is very meagre, although diameter growth continues to go on very rapidly. In my studies of the subject I have always regarded the beginning of the rest period as being the time when all cell division ceases. The other activities, such as respiration and molecular changes, undoubtedly continue, but I have regarded them as vital phenomena which always go on as long as life exists, regardless of whether growth is taking place or not.
The above has reference to the winter rest, usually known as the dormant period. In herbaceous plants-biennials and perennials, there is a similar cessation of growth during winter in temperate regions, the time of beginning and ending, as in woody plants, being chiefly determined by altitude, latitude and season.
The rest period in bulbs is especially long and deep. The natural period of dormancy in bulbs begins at the close of their comparatively short season of growth, whenever that may be, whether in summer, fall or winter. The rest ends, under natural conditions, after a long period of inactivity, followed by good growing conditions.
REST PERIOD EXPERIMENTS.
It is a matter of common experience that many of the plants which rest or remain inactive during the winter, will begin to grow in a short time if taken into a warm room, even in midwinter. Some will even begin to grow outside during the winter after the weather has been mild for a few days. On the other hand, there are many plants that will make no growth whatever until near spring, even though they are taken into a warm, moist room and kept there. These experiences led to the popular belief that there are many plants which have such profound resting periods that they cannot he made to grow until at or near the normal time for awakening in spring.
In order to learn something of the fundamental nature of the rest period and to test the extent to which plants may be caused to start into growth out of their season, i.e., while dormant, nearly three hundred species of woody plants were examined by bringing them into the greenhouse after they had gone into their resting state, and noting which ones began to grow under the influence of the warmth. As anticipated, it was found that the dormant state varied very greatly with the different species, not only in point of the duration of the rest, but also in the degree of its intensity. The extremes in woody plants are probably represented by two typical cases, viz., Spiraea sorbifolia, L. and Fagus sylvatica, L. The former is a plant which appears to have no resting period, seeming to stop growth only when the weather is actually too cold for cell division, while the latter goes dormant in the fall at the approach of cold weather and remains so until the time of settled weather in spring, even though it be brought into warm, moist room. In short, the Spiraea will make an almost continuous growth during the winter when kept inside or will spring into growth within a day if brought in during the winter, while the Fagus will not grow at all during its period of rest through the influence of warmth alone.
Out of the long list experimented with, quite a number were found to be very difficult to awaken from their resting state, even under severe treatment, while others would eventually make some growth merely under the influence of warmth, but would grow much more readily when given special treatment. Experiments were then inaugurated to test the efficacy of certain treatments for forcing growth.
It is commonly said of seeds that they will not germinate, in many instances, until after they have rested—that is, remained in storage for a few weeks after ripening. To secure information on this subject about one hundred and twenty-five different species of seeds were tested. Beginning in early summer, seeds of everything available were gathered as soon as ripe and planted immediately. Some of each were dried thoroughly and then planted. Also whenever practicable to do so specimens of each species were gathered before they were quite mature and planted at once.
Generally speaking, seeds of herbaceous annuals, biennials and perennials will grow and show a high percentage of germination when planted immediately after ripening without giving them an opportunity for drying out, even in the pods or other containers while attached to the parent plant. After ripening and being dried from one to six days, depending upon the character of the seed coat, the seeds will grow in a majority of cases, but not so quickly as those that were not permitted to become dry.
The behavior of seeds of woody plants, whether of trees, shrubs or vines, were all much alike in that few of them would grow either. when gathered as soon as possible after ripening or after being dried from one to six days. In the few instances where both would grow, as for example in the case of Morus alba and Gleditschia triacanthos, the former made the quickest and best growth where seeds were taken and planted immediately upon ripening, and in the latter the results were just the reverse. In other cases as the Robinia pseud-acacia and Quercus alba only the dried seeds grew. The seeds of shrubs, for the most part, failed to make any growth; both where planted immediately after ripening and after being dried for a few days. Therefore, it would seem that the seeds of herbaceous plants do not have a resting period necessarily, as they are able to germinate, both when freshly ripened and after they are thoroughly dried out. The character of the seed coat will probably be the most important factor in causing delayed germination.
As a result of extensive studies and experimentation, it is concluded that nearly all of our ordinary woody plants, such as fruity trees, ornamental shrubs, etc., have definite resting periods—that is, that each species, and possibly each variety, has a certain number of days during which it will not grow under natural conditions. These facts have led to the conclusion that a knowledge of the so-called winter rest period of plants, its nature and extent in different kinds and types of plants, is fundamental to the study of many of the practical questions of horticulture.
One practical application of the above principle is that, if, say, in the case of the peach, which is known to have a comparatively short rest period (about six weeks), the trees can be caused to continue growing later in the autumn—that is, go dormant late, they will remain dormant a correspondingly longer time in late winter, and hence not be so liable to injury by freezing after having made a slight growth.
In Missouri peaches are nearly always killed by reason of their habit of premature growth during warm days in late winter. Every fruit-grower knows that there is no danger of growth taking place in early winter, no matter how warm and mild the weather may be, because the trees then are in the midst of their rest period. When fully dormant, peach buds can sometimes safely withstand the low temperature of 15° F., but after having made a slight growth, zero weather will nearly always be sufficient to kill them all.
The subject of hardiness, then, in the peach, as well as in other plants, may be found to be intimately associated with the rest period. If it were known definitely that plants have a tendency to grow at unseasonable times by reason of the fact that they have very slight resting periods, or that the resting stage is at an end, we might then set about devising means of regulating the period of dormancy, or, knowing that their habits of growth in this respect cannot he easily changed, of finding means for protecting them.
Another practical advantage resulting from a knowledge of the rest period of plants, together with a practical means for rousing them into growth out of season, is in the commercial forcing of flowering plants and garden vegetables, as practiced by the florist and gardener. Large sums of money are now invested in plants like lilac, deutzia, etc., they being grown especially for forcing purposes in winter. The use of anaesthetics (ether and chloroform), as well as freezing and desiccation, in causing these plants to grow quickly and bloom profusely, will undoubtedly soon play a very important part, if indeed they do not supplant other methods of forcing. Vegetables, like rhubarb and asparagus, have been forced by the use of anaesthetics in a commercial way, to only a comparatively slight extent, although with the improvement in methods of handling, anaesthetics and also desiccation and freezing may be used to a much larger extent in this sort of forcing work.
EXPERIMENTS WITH THE REST PERIOD IN WOODY PLANTS.
The first experiment was to test the effects of warmth alone in causing woody plants to grow during the early stages of their winter rest. In mid-autumn 234 species of trees and shrubs were collected and placed in a warm, moist greenhouse. Under the influence of the warmth 125 grew and 109 did not.
In mid-winter another lot, consisting of 283 species, was removed to the greenhouse and 244 grew and 39 did not.
Since many kinds failed to grow in the two tests and a large number made but a slight growth, another set was collected late in February. There was a total of 63 kinds in this list. Forty-nine began to grow within nine days and all but two of the remainder grew in less than three weeks.
The final test, consisting of ten species, collected March 17, represented mainly those that had not grown before. Nine of these grew.
These tests showed that the rest period is more profound during its earlier stages than it is later and that the plants were able to overcome their dormant state with greater and greater ease as the season advanced.
The next step was to find if the plants could be forced to make growth earlier than they would do so merely under the influence of warmth. The following treatments were given to force growth: etherizing, freezing, desiccation and confining in a warm, moist, dark-chamber. These were tried singly and in combination at different times during the fall and winter.
The marked effects of the ether were seen in the larger percentage that grew when treated, and especially in the shorter time required for growth to begin. A gain in time of growth of more than ten days followed the 48+48 hours and the 72 hours' treatments, while there was but 6.5 days gain from the 48 hours' treatment. Etherizing for 48+48 hours gave the best results in this test. (Forty-eight plus 48 hours means that plants were exposed to ether fumes 48 hours, removed for 48 hours, and then again subjected to the fumes for another 48 hours.)
In early winter another test was made. Ether alone having been found to be effective in awakening many plants from their dormant state, two new methods of treatment-freezing, and confining plants in a moist, dark-chamber—were introduced. It was desired also to find if the ether would exert any more influence when used immediately after the plants were severely frozen than would the freezing alone. The dark-chamber supplied two conditions not present in the greenhouse, viz., uniform temperature and high, constant humidity.
Thirty-five species were treated as follows:
Of the thirty-five species treated all but three grew. Ninety-seven and seven-tenths (97.7) per cent, grew from one or more treatments, while only 65.7 per cent. grew without treatment. Those frozen 7 days then etherized 48 hours, made the quickest growth, average 10.3 days, a gain over the control of 7.7 days. Freezing for one week and for three weeks caused about the same results in length of time required for growth and the percentage that grew. Comparing those frozen 7 days and those similarly treated and in addition etherized 24 hours, it is found that the addition of the ether did not hurry the growth, but the increase in the number of those that grew was more than 11 per cent.
Comparing one that was frozen 21 days with one frozen for a similar period followed by etherizing for 48 hours, similar treatments except for the ether, the anaesthetic seems to have exerted an extraordinary effect in two ways; first, the time was much shortened, and second, a smaller percentage grew.
In every treatment except the one by freezing 7 days and etherizing 24 hours, a smaller percentage grew than-was the ease with untreated plants. On the other hand, every treatment but one-in moist chamber 17 days—caused a quicker growth than the control. It is also noticed that the treatment giving the shortest time for growth (frozen 7 days and etherized 48 hours), shows the smallest number that grew, loss 20 per cent! This is true in less degree with nearly all of the treatments. Evidently freezing, followed by strong etherization, exerts a profound influence on dormant plants, causing them either to grow and grow quickly or to be killed or severely injured. There was little evidence of any of the plants remaining indifferent to the treatments. Those species that grew without treatment and also many that forced easily, were often injured by the severe treatments. In other words, they responded readily to the treatment, but when prolonged, many were killed.
The last of the three main sets of treatments, which was by far the largest of them all, both in number of species used and the number of treatments employed, was given in early winter. Besides the control there were sixteen separate treatments in this experiment, as follows:
Freezing for 8 days, followed by etherizing for 72 hours caused the quickest average growth, 9.9 days, a gain of 11.6 days over the control. Severe drying (for 5 days) was as effective as etherizing for 48 hours. Freezing for 8 days produced a quicker growth than drying for one or two days.
The highest percentage of growth followed the use of the dark-chamber for 8 days, then etherizing for 48 hours, a gain of 7.2 per cent. over the control. The growth was slowest following the use of the dark-chamber alone, loss of one to two days. Results from drying are interesting. The longer they were dried the smaller the percentage that grew; and, with one exception, the longer they were dried the quicker they grew. All things considered, etherizing for 48 hours gave the best results. The value of the methods of treatment for forcing in the order of their worth runs as follows: First, ether (time 12.9 days) ; second, freezing (time 17.3 days); third, drying (time 18.4 days), and fourth, dark-chamber (time 23.3 days).
REVIEW OF EXPERIMENTS AND RESULTS.
At the beginning of the experiments it was known that plants have a dormant period under natural conditions where growing out of doors. Taking these as they were found the first question to be answered was, which ones could be aroused into growth during the early stage of their dormancy, by any means whatsoever. In a general way, it was known that many woody plants would grow with comparative ease, merely under the influence of warmth, while others would make no growth whatever under such circumstances. Also some data were available showing that ether and chloroform had been found to be effective in hastening growth in the few cases where they had been tried on a comparatively small number of plants, but to what extent these or any other treatments would hasten growth or produce growth at all with the great majority of plants was practically unknown.
Of the 283 species that were merely brought into the greenhouse, but not otherwise treated, 211 grew in January or earlier, 36 by February or before, and the remainder by March or earlier.
In all, during the three main experiments, from mid-autumn until mid-winter, 133 different species were treated to force growth and all but 14 grew. By various additional means, not recounted here, 7 out of the 14 were caused to grow, so that growth was secured in January or earlier from a total of 126 species. Of the 7 species which remained, Liriodendron tulipifera and Fagus sylvatica grew in February, and Carya aquatica, C. porcina, Juglans regia, Quercus alba and Q. coccinea in March.
The real importance of this information lies not so much in the matter of whether the plants grew with or without treatment, as in the fact that they would grow at all by mid-winter or before. This shows conclusively that the great majority of species, indigenous to temperate climates, do not have a firmly fixed winter resting period from which they cannot be awakened.
Experience in this investigation has shown that there are apparently times when a given species will grow more readily than at other times under treatment, which leads to the belief that if the treatments are repeated at frequent intervals it is highly probable that all of the species may be forced into growth in early winter or in mid-winter. From the experiments of Klebs we are justified in the opinion that with previous cultural treatment the most resistant forms can be made to grow at any time and perhaps eventually be made to have very short, or light, resting periods, if not to omit them entirely.
EFFECTS OF ETHER, FREEZING, DRYING, ETC.
The whole question of how vapors, freezing and desiccation, cause growth to begin in dormant plants that are already in a growing condition is as little understood as the rest period itself. The first experiments of the effects of ether and other vapors and gases on plants were made in connection with studies of respiration, transpiration and movement. Such experiments date back for about fifty years. Beginning in the early seventies and continuing later, numerous experiments were made where anaesthetics were extensively used, but almost altogether in connection with the study of plants that were in a growing condition. Some little attention was given to treating seeds and spores, but no reliable results are available. In the year 1900 Johannsen aroused a widespread interest in the use of ether and chloroform for forcing flowering plants into growth for commercial purposes. Regarding the workings of ether and chloroform he declares that they could be only in two ways, that by stimulating the "power of growth" or removing the "hindrance," or, possibly, by both. He does not believe that the effects of ether, freezing and drying are the same, even though the results are similar. In fact, he cites an experiment in which the drying of barley seed had no effect, while treating with a very small amount of ether caused them to grow quickly, thereby disagreeing with Dubois, who reports that ether exerts a strong dehydrating effect and is therefore similar to freezing and drying.
It has been found that ether increases the respiration, but this in itself does not constitute growth. Latham found that in chloroformed fungi the increased growth was accompanied by less acid formation and less sugar consumption, thus indicating increased economy in metabolism. Gerrassimow studied the effects of ether on Spirogyra and concluded that the increased growth was caused by the anaesthetic having stimulated the cell nucleus.
Aymard gave it as his opinion that anaesthetics produce the same effects on vegetation as freezing or drought; that they act on vegetable tissue as dehydrating agents. As proof of this, he found that roots of the lily-of-the-valley placed under a bell-jar with phosphoric anhydrid and other drying liquids, forced even more quickly than when submitted to ether vapor. It was also found that etherized plants lose considerably in weight.
Because of the accumulation of sugar in potatoes which had been kept in storage at low temperatures, so conclusively proven by Muller-Thurgau, the writer, as well as Klebs, came to the conclusion that growth in woody and perhaps all other plants is always preceded by sugar formation. From this premise it seemed logical to go a step farther and decide that it was necessary for sugar to be formed before growth could take place and that the specific effect of ether and the other treatments that produce growth, was to render the service of making said sugar or sugars available.
To get at the facts in the case the writer had quantitative chemical analyses made of the sap of peach and apple twigs at intervals of ten days from November until May. The determinations were made for reducing sugar. The results showed that the sugar content in the peach constantly increased until the last of December, when it began to decline and kept growing less until May 1st, at which time the experiment closed. The same was true of the apple twigs, except that in this case the maximum amount was reached about six weeks later—that is, February 1st.
These results show conclusively that sugar formation does not precede growth and therefore it is clear that if the ether, etc., exert any specific influence it is not in the direction of making sugar available. It may be that these results are defective in that the reducing sugar was not the only sugar present. To settle this point, samples were collected and determinations made at frequent intervals from fall until summer for total sugars. These determinations were made during the past year, but at this writing the figures are not yet available. However, the indications are that the first results will not be materially changed. Next year it is planned to subject dormant twigs to different treatments for forcing growth and then make chemical analyses for sugars, proteids, etc., at once and at intervals until growth begins.
It is strongly suspected that certain proteid changes are necessary to start growth, but nothing definite is known about this. Cold will precipitate certain proteids, but Gorke says that the proteids in each species of plant may be different, so that any reasoning as to conditions in peaches and apples would have to be by analogy and therefore the results would be entirely too risky to be dependable.
It is popularly supposed that drying is very similar to freezing, as both remove water from the plant body and the latter, at least, quickly condenses the cell sap. They may, however, not be identical in their effects.
In conclusion it may be added that whether etherizing, drying and freezing are alike or related in the nature of their workings or not, the experimental results herein recorded show that all will break the winter resting period of plants. However, it is very probable that none of the agents used for forcing growth exert any specific action, but are rather the means, possibly in distinctly different ways, of setting into activity directly and indirectly, certain complicated vital machinery which results in growth.
COLONEL BRACKETT: The paper is now before you for discussion.
PROF. REIMER: I would like to say that this is a very important matter, and has a more practical bearing than some of us may suppose. For example, fig culture in the upper South depends entirely on the rest period, and the only varieties of figs we can grow successfully there are those that have a long resting period. We can determine exactly what varieties will succeed there, and what varieties will fail, by determining the amount of time and heat required to bring these plants into activity. The only varieties successful are those in which the rest period is not easily broken, and there are very few varieties which will succeed. Nearly all varieties of figs become active very early, and are not successful in the upper South for that reason.
PROF. CRAIG: I didn't quite gather whether in the ease of those plants where every means employed failed to cause growth, Prof. Howard was working with cuttings, detached parts of the vegetable body, seeds, or the entire plant. It would make an important difference with a good many of these plants, as to whether parts of the organs or entire plants were used. We know positively that in some of the genera of plants, as Quercus, the oak tribe, that if we try detached parts, even under the beneficient effects of ether, or the application of cold, they remain absolutely dormant; but if they are attached to a root, and we can get water into the root, we can get growth without either ether, chloroform, or any other agent; my question is, in those plants which remained obstinate. what parts did you use?
PROF. HOWARD: We used the branches, consisting of the one, two, and three years' growth, in nearly all cases; these were two or three feet long.
PROF. CRAIG: Some fruits, like the gooseberry, which is the product of a short summer season and a long resting season, and therefore the product of that environment, cannot be hastened until they are ready to grow.
PROF. HOWARD: I never tried the gooseberry. For three years I have had tulip trees, white oak and beech, together with some peach trees, growing in the greenhouse in good, rich soil, specially prepared to receive them. They have not been frozen in three years. They become dormant in winter, then they grow again. At the present time they are all dormant, they have not grown since about September. The white oak is showing signs of growth, but the tulip and beech are apparently just as dormant as though they were outdoors. I expect all will begin growth about March. After that they will have two or three seasons of growth and dormancy before winter again.
PROF. LAZENBY: The commercial growers of rhubarb for forcing in Ohio make very great use of frost, and find it very valuable. It is not seemingly necessary to force rhubarb early in the season. By plowing out the roots and exposing them, they freeze much more readily than in the ground. After a frost of that kind, they can be taken right into the greenhouse and forced very readily. Without a frost they cannot be forced. I have been very much interested in what might be termed the resting period of trees with reference to seeding, more particularly our forest trees. It doesn't seem to me that trees have a resting period as regards seed production. One is made to believe that a tree depends largely upon accumulation of starch in its tissues before we have a full fruiting period. If you cut off a twig just below where the blossoms occur, you will find an abundance of starch. You are not likely to have very much growth. I think this seeding seemingly in cycles is due largely to this accumulation of starchy material. We have never had such a dearth of tree seeds in central Ohio as the present year. Black walnuts, butternuts, beech nuts and hickories were completely wiped out. That was due to climatic conditions to a considerable extent.
PROF. ROLFS: In connection with the period of summer growth, I have noticed that the black walnuts which are not really indigenous to the- peninsular portion of Florida, begin shedding their leaves very early in the fall. I think the point Prof. Howard has brought out rather explains this. The seeds come from farther north. When brought into peninsular Florida the trees begin shedding their leaves just when their northern relatives are shedding, although they have grown here for thirty or forty years They seem to have inherited the tendency of dropping their leaves at that time. I was interested in what Prof. Howard said about the butternut, a very close relative of the black walnut.
PROF. HOWARD: I would say that in the case of the black walnut we found very marked differences in the dropping of leaves from trees which grew from seeds that were brought from Alabama on the south and Ontario on the north. I don't recall the number of days. The same was true of the box elder grown in our own State of Missouri, and trees from seeds that came from Nebraska. Those northern plants in both cases shed their leaves a month earlier, and the southern ones kept on growing in some instances until overtaken by frost. There was severe winter injury as a result of freezing in immature condition. In the case of honey locust from Alabama, the seedlings grew, until frost came and they were frozen back to the ground. They had made a growth of five or six feet in one year. The second year they did the same thing. The third year the winter was milder and they lived through. Occasionally after that they were killed back. Now they are perfectly hardy, and are trees thirty or forty feet high. As botanists would say, they have adjusted themselves to their changed environments.
COLONEL BRACKETT: Honey locust is known to stand a temperature of twenty below zero, still it was not hardy with you.
PROF. HOWARD: It was not hardy the first three or four years after coming from the seed secured in Alabama. After that it was hardy.
PROF. CRAIG: It is an important question how far you could carry that principle. I mean the possibility of plants adjusting themselves to their environments. An interesting example of the possibility of a certain type of adjustment is seen in Ottawa, Canada, where northern forms of bald cypress, entirely tender in that locality, are grown. But instead of being grown as towering trees, as in their native home, they are grown as low shrubs. It is an attempt on the part of the plant to fit itself into its surroundings. I seriously doubt whether there is any real change in the constitution, or the hardiness of the plant. I question whether that adjustment means a change in its ability to stand the cold. I don't believe the innate constitution of the plant may be changed.
PROF. HOWARD: I believe there is a good deal of truth in that. They were in a rapid growing condition when they were overtaken by frost in their early years. After nine years they were still growing late into the fall, then gave up the struggle. As to the application, I have given very little attention to that. I certainly hope that many others will be able to make use of some of the facts that we have collected, the experimental data that we have secured, and be able to apply them to practical problems in horticulture under varying conditions. The hardiness of the peach is one direction in which we hope to make a practical use of what we have learned about the dormancy of plants. We want some basis to work from, then we hope to be able to take another step. After all, we never know just when the scientific fact is going to be useful in practice. I think some of these facts can be made useful.
PROF. ROLFS: We are making very large use of this particular fact. We know that the Citrus trifoliata is a deciduous member of the citrus group, and consequently has this dormant period strongly marked. By budding one of the species of Satsuma upon that stock that has this dormant tendency, more or less of this dormant tendency is imparted to the Satsuma, consequently the Satsuma on the trifoliata stock is hardy much farther North than on other stocks. In Texas the Satsuma is being planted out on trifoliata stock in hundred-acre blocks.
PROF. REIMER: There is one other factor involved. For instance, it is usually claimed that the apple will not grow in the warmer sections of the South, because when brought to the warmer sections, it drops its foliage about mid-summer, and the tree practically stops growing. But we have found that this is due largely to fungous diseases on the foliage, which are worse in the warmer sections of the South; and we find that by spraying these same trees thoroughly with fungicides, they can retain the foliage until Christmas time, and the trees keep on growing. I believe with some plants under certain conditions that is a factor which should be kept in mind.
PROF. LAZENBY: These two illustrations do seem to show that it is a question of environment. It does seem to me that as far as real acclimatization is concerned, there is very little to he said. I sometimes doubt whether there is such a thing as acclimatization of any particular species. Of course, by change of environment, no doubt good results may be effected; but I cannot see that the peach or any of these plants that have been cultivated for years have any change in inheritance. I may be wrong, but I have come to the conclusion that there is no such thing as acclimatization of a tree.
PROF. ROLFS: You don't claim that for the progeny?
PROF. HOWARD: No, not at all, but for the tree itself.
Howard: Second Report April 1915
Howard: Third Report April 1915
Howard: Fourth Report April 1915
Howard: Fifth Report June 1915