Plant Physiol. 1927 July; 2(3): 273-286.
RELATION OF COMPOSITION TO GROWTH AND FRUITFULNESS OF YOUNG APPLE TREES
AS AFFECTED BY GIRDLING, SHADING, AND PHOTOPERIOD1

R. H. ROBERTS
(WITH TWENTY-TWO FIGURES)
1Published with the permission of the Director of the Wisconsin Agricultural Experiment Station.

Introduction

An effort is being made to record the relations of internal composition of apple trees to their vegetative responses, especially to the particular growth characters accompanying blossom bud formation. Various environmental conditions are being used to vary the plant composition. In a previous report (4) the effects of nitrogen fertilization were considered. It was observed that the vegetative condition as measured by such characters as length of growth, leaf colors, bark colors, and plant composition varied directly with the nitrogen fertilization under the cultural conditions provided: dwarf trees in pots in the greenhouse. On the other hand, blossom bud formation was not correlated to cultural treatment, but rather to growth character and to a "balance" in composition when the carbohydrate and nitrogen content are compared. This is a condition similar to that described by KRAUS and KRAYBILL for fruit development in the tomato (2). The most vegetative plants or growths were highest in nitrogen, lowest in carbohydrates, and unfruitful; the least vegetative plants were lowest in nitrogen, highest in carbohydrates and were unfruitful; and the moderately vegetative plants were intermediate in nitrogen and carbohydrate content and were fruitful.

The significance of the above relation of composition to blossom bud formation and fruiting lies in this: if reproduction is related to growth character, which is the response to internal composition, rather than directly to cultural treatment, then, (1) we are provided with a basis for interpreting the conflict of results secured from the limiting factor or plot method of investigating tree performance, and (2) we can use the growth character of the tree as the basis for diagnosing orchard conditions when attempting to decide upon needed cultural treatments. Instead of using the present unsuccessful method of trying to duplicate a cultural treatment which is reported as being profitable in another locality, the practical grower can attempt to produce by any feasible means and with a reasonable assurance of success, such amounts and types of growth as give the desired fruiting responses. To illustrate, the use of readily available nitrogen fertilizers has become very general in recent years, because of the frequent increase in fruiting which results. In situations where low production is due to an over-vegetative type of growth, nitrogen fertilization tends further to delay or reduce production. A similar condition pertains as regards pruning and cultivation; these may be harmful as well as beneficial to fruiting, depending upon the vegetative condition of the trees being considered.

Materials and methods

The data presented herein were collected to determine the effect of three other environmental conditions upon tree composition, blossom bud formation, and to further test whether or not fruitfulness was related directly to the treatment or to the balance in composition produced by the treatments. The conditions used were girdling, shading, and reduced photoperiod.

As regards shading and girdling, one hundred yearling Wealthy trees grafted on standard seedling stocks were planted in early May, 1926. When the new growth averaged one to two inches long, one half of the row was covered with a burlap shade on a lath frame. Further, one half of the shaded and of the unshaded trees were girdled by removing a half inch of bark about mid-height. The girdle was covered with grafting wax. Several girdled trees died. Many girdled-and-shaded trees died. The trouble was obviously due to lack of root development. No trees were lost from girdling in late July after appreciable root development had occurred. Some sucker growth was produced below the girdles. A few trees produced sufficient callus to bridge the girdle late in the growing season. These latter were avoided when sampling for chemical analyses. The effect of shading is probably less than would ordinarily obtain, as the sunshine from June to November was only 41.5 per cent. of possible or 69.4 per cent. of normal in 1926.

Results

The growth produced under the different treatments is shown in table I. The growth data were obtained from averages of paired trees. That is, the "check" is made up of trees having a previous season length and weight like the treated trees. Thus each treatment has a separate cheek. There is considerable difference in cheeks because the girdled trees which died were smaller than the ones which lived and because the larger cheek trees tended to grow more than the smaller ones.

There are some items of especial interest in table I:

1. The greatest amount or quantity of growth when measured by per cent. gain in weight of tree, total length of shoots, or amount of new roots was made by the ungirdled sun trees. The least was made by the girdled, shaded trees.

2. On the other hand, the extremes of kind or quality of growth when measured by such characters as individual shoot length, diameter and internode distance are found in the shaded trees and the girdled trees in the light.

3. The differences in character of growth above and below the girdles, as will be pointed out later, make an error in measuring total growth of tree. For instance, the seasonal increase in weight of shaded and girdled trees (average 10.8 gm.) is less in some cases than the increase above the girdle, thus showing a loss of weight below the girdle.

TABLE I
EFFECT OF SHADE AND GIRDLING UPON GROWTH OF YOUNG APPLE TREES

TREATMENT INCREASE
IN
WEIGHT
LENGTH
TERMINAL
BRANCH
DIAMETER
TERMINAL
GROWTH
INTERNODE
LENGTH
TOTAL SHOOTS RATIO TOTAL
SHOOTS TO
PER CENT.
WEIGHT
INCREASE
GREEN
WEIGHT
OF NEW
ROOTS2
NUMBER LENGTH
  Per cent. cm. mm. mm.   cm.   gm.
Sun, ungirdled 162.9 37.2 2.86 24.5 3.35 97.0 0.60 7.43
Shade,                
ungirdled 51.4 40.4 2.51 30.0 2.93 85.2 1.66 2.67
check 135.6 32.6 2.69 23.6 2.93 71.3 0.53  
Sun,                
girdled 40.6 12.1 3.40 20.7 3.66 52.2 1.29 2.48
check 154.5 40.9 2.68 23.1 3.00 86.3 0.56  
Shade,                
girdled 19.6 21.2 3.08 27.4 3.20 49.21 2.51 0.49
check 168.5 36.7 3.02 25.8 3.40 104.3 0.62  
1Less branching as well as less growth of suckers than sun and girdling.
2Average of three trees.

The failure to differentiate between the amount and character or quality of growth is believed to be a frequent error in interpreting tree growth data secured from cultural plots.

Further details of growth character as affected by the shading and girdling are presented in tables II and III and figs. 1 to 19.

Some responses of particular interest, most of which are recorded in the tables or illustrations follow:

FIG. 1. Character of shoot and foliage growth: Left to right, 1. Sun; 2. Shade; 3. Sun and girdled; 4. Shade and girdled. The leaves of 1 and 4 are very similar in texture although appearing different due to a difference in orientation.

TABLE II
EFFECT OF SHADE AND GIRDLING UPON FOLIAGE DEVELOPMENT OF YOUNG APPLE TREES*

TREATMENT LEAF
SIZE
LEAF THICKNESS CELLS PER
SQ. MM.
STOMATA
PER SQ.
MM.
MILLION CELLS PER
LEAF SURFACE
ABOVE
GIRDLE
BELOW
GIRDLE
UPPER
SURFACE
LOWER
SURFACE
LOWER UPPER
  sq. in. mm . mm.          
Sun 4.08 0.276 0.272 1737 1920 583 4.57 5.06
Shade 6.12 0.153 0.161 1385 1895 460 5.46 7.46
Sun, girdled 2.18 0.289 0.255 2158 2772 820 3.03 3.90
Shade, girdled 5.00 0.158 0.145 1961 2468 580 6.32 7.72
*Details of leaf and stem structure recorded by MARIAN DEATS ABEGG.

TABLE III
EFFECT OF SHADE AND GIRDLING UPON STEM ANATOMY IN YOUNG APPLE TREES

TREATMENT NEW GROWTH NEW XYLEM
RADIUS RATIO NUMBER CELLS
IN RADII
TRUNK ROOTS
PITH XYLEM BARK* PITH
——
XYLEM
PITH
——
BARK
XYLEM
——
BARK
PHLOEM RAYS XYLEM UPPER LOWER
  mm. mm. mm.             mm. mm. mm.
Sun 0.622 0.306 0.610 2.03 1.02 0.50 15 8 18 0.68 0.74 0.63
Shade 0.585 0.263 0.479 2.22 1.22 0.55 11 7 13 0.23 0.38 0.24
Sun, girdled 0.620 0.502 0.722 1.23 0.86 0.70 18 18 39 0.81 0.34 0.16
Shade, girdled 0.577 0.381 0.565 1.51 1.02 0.67 13 9 20 0.30 0.07 0.04
* Phloem and cortex
  1. The influence of the treatments upon diameter growth.
  2. The marked reduction in secondary thickening in the shade and below girdles. Bast fibre development was also nearly absent. Only the check trees had second year bast in the lower trunks.
  3. Ray cell shapes varied greatly as well as vessel, wood-fibre, and wood-parenchyma cell sizes.
  4. Starch grain sizes and storage cell wall thickenings were markedly affected.
  5. Leaf palisade was strikingly reduced by shade.
  6. Large leaves on shade trees resulted from both larger cells and greater number per leaf.
  7. Girdling and shading greatly reduced root development. This is correlated with the effect upon new xylem production.

Notes:

  1. Three trees were used for each sample.
  2. A modified Kjeldahl was not used in determining total nitrogen, as apple wood is rarely found to contain an appreciable amount of nitrates.
  3. Ptyalin digestion was used in extracting starch.
  4. The reducing substances were obtained by the Munson-Walker method and were measured by the Shaffer-Hartman titration method. The volumetric method gives results with mature apple wood comparable to the gravimetric determinations.
  5. Acid hydrolysis was accomplished by refluxing with 2.5 per cent. sulphuric acid for one hour.
  6. The bark was not analyzed separate from the wood. This may be a source of error. In general, however, it seems that bark and wood analyses vary more in percentages, due obviously to a greater inert fraction in the wood, than in the direction of difference. While they may show decided differences in percentage the fluctuations in amounts are parallel.

Table IV shows the usual lack of relation between the starch fraction and either the reducing or acid hydrolyzable fractions. As usual, these latter are not correlated with vegetative condition or anatomical structure.

The relatively high starch content below the girdles in unshaded trees is obviously due to slight utilization of reserves, the small top growth made before the time of girdling being from material near the growing points and from newly synthesized foods.

The intermediate carbohydrate content and nitrogen carbohydrate relation of new shoots on sun and shade-and-girdled trees as compared to the shaded and sun-and-girdled trees is in agreement with their intermediate growth character.

The low nitrogen content of the wood and roots below the girdles, especially in the shade, was a surprising condition. The growth character of the suckers arising from this region were of a characteristic "high nitrogen" appearance. As there is a greater reduction in carbohydrate percentage than in nitrogen percentage below the girdles, the lower portion of the stems do have a "high nitrogen" condition. This is taken as evidence that growth character is determined more by the relations or qualities of foods than by the actual quantities present.

The low nitrogen content below the girdles may be associated with the poor root development on girdled trees rather than being a result of girdling, although the nitrogen content of the top is not related to root extension either on a percentage or absolute basis.

TABLE IV
EFFECT OF SHADE AND GIRDLING UPON COMPOSITION OF YOUNG APPLE TREES

SAMPLE TREATMENT MOISTURE SPECIFIC
GRAVITY
TOTAL
N
STARCH REDUCING SUBSTANCE ACID
HYDRO-
LYZABLE
TOTAL
CARBO-
HYDRATE
RATIO TO N
FREE TOTAL STARCH TOTAL
CARBO-
HYDRATE
    Per cent.   Per cent. Per cent. Per cent. Per cent. Per cent. Per cent.    
One year
wood
Sun 52.1 1.115 0.876 7.40 3.84 6.53 6.05 19.98 8.45 23.12
Shade 54.9 1.084 0.869 6.61 3.37 7.76 3.73 18.10 7.61 21.08
Sun, girdled 50.2 1.356 0.775 9.92 4.13 8.28 7.65 25.85 13.15 33.40
Shade, girdled 51.3 1.207 0.953 8.88 3.50 7.48 4.31 20.67 9.32 21.70
Two year
wood,
upper half
Sun 46.3 1.106 0.557 5.82 3.57 5.14 4.08 15.04 10.46 27.10
Shade 48.1 1.008 0.668 5.07 3.46 4.92 4.66 14.65 7.58 21.95
Sun, girdled 46.6 1.056 0.540 8.29 3.89 6.28 6.48 21.05 15.36 38.99
Shade, girdled 48.3 1.009 0.582 7.40 5.52 7.75 5.48 20.63 12.72 35.50
Two year
wood,
lower1 half
Sun 44.7 1.005 0.528 4.97 1.81 3.13 2.99 11.09 9.40 21.05
Shade 47.1 1.042 0.526 4.47 2.26 3.23 4.38 12.08 8.49 23.00
Sun, girdled 48.6 1.010 0.443 4.57 1.80 3.50 4.04 12.11 10.32 27.35
Shade, girdled 52.6 0.961 0.376 0.92 1.47 2.97 2.96 6.85 2.45 18.23
Two year
roots
Sun 55.2 1.049 0.813 14.20 4.13 6.11 4.29 24.60 17.50 30.30
Shade 56.8 1.002 0.963 12.42 4.16 6.32 5.66 24.40 12.95 25.38
Sun, girdled 55.9 0.983 0.757 15.98 4.22 6.71 5.54 28.23 21.12 37.30
Shade, girdled 59.2 0.977 0.718 4.02 4.93 8.42 6.32 18.76 5.59 26.12
1 Below girdle in case of girdled trees.

TABLE V
NITROGEN AND STARCH IN YOUNG APPLE TREES*

TREATMENT NEW GROWTH TWO YEAR WOOD TWO YEAR
ROOTS
UPPER HALF LOWER HALF
    mg. mg. mg. mg.
Nitrogen Sun 19.64 22.73 17.98 19.00
  Shade 11.36 32.68 30.18 23.45
  Sun, girdled 13.86 21.60 20.32 19.75
  Shade, girdled 16.75 29.30 8.98 8.68
Starch Sun 165.7 237.8 169.2 332.1
  Shade 86.7 247.6 256.2 337.5
  Sun, girdled 177.1 331.9 209.6 417.0
  Shade, girdled 156.1 373.0 21.2 48.7
*The crown piece, including some stem and the main root as well as the new fibrous roots, are not
included. Differences in previous season's bulk make direct comparisons of treatments useless, but
the general tendencies where chemical differences are large can be clearly seen.

TABLE VI
EFFECT OF SHADE AND GIRDLING UPON THE PERCENTAGE OF BLOSSOM BUD FORMATION IN YOUNG APPLE TREES

TREATMENT SPURS TERMINALS LATERALS TOTAL BUDS
Sun 0.0 0.0 0.0 0.0
Shade 0.0 0.0 0.0 0.0
Sun, girdled 47.8 100.0 65.7 65.2
Shade, girdled 1.2 5.6 0.0 0.8

Two conditions will be noted from table VI. One is the relation of blossom bud formation to composition and growth character, and not directly to treatment. That is, girdled trees in the sun produced abundant blossom buds, but when such trees are shaded they are almost unfruitful as are the sun, ungirdled trees which have a bike composition and growth character.

It will also be noted that blossom bud formation accompanied a condition of least growth and greatest carbohydrate content. This is because the very high carbohydrate, under-vegetative type of tree was not represented in the present series. The blossoming of trees of intermediate or ''balanced" composition will be found in the following report upon the effect of reduced photoperiod upon apple tree performance.

It was desired to determine whether a "short day" had a direct effect upon blossom bud formation or produced its effects through changes in the plant composition. Consequently, trees which were in very different vegetative conditions due to previous variations in nitrogen nutrition were grown with both a full and a six-hour photoperiod from March to August, 1925. The trees were grown in the greenhouse in 12-inch earthen pots resting in trays for sub-irrigating. A ventilated ''cage'' constructed of slats and building paper was placed over the short day trees from 2 P. M. until 8 A. M. each day. Only six trees were grown in each lot. Because of the pronounced and consistent behavior of these trees (fig. 20), a record of their performance is given here.

The chemical composition on August 6, is shown together with the blossoming observed in early 1926, in table VII. Sampling was at approximate maturity of the long day plants. The short day trees with a high nitrogen nutrient definitely stopped vegetative extension only when water was withheld. They gave no sign of "maturing" as did the other trees.

There was a marked increase in length of growth. Variations in leaf and stem anatomy were striking. These will be reported in detail after repeating the photoperiod experiment with a larger number of trees.

Very clearly, a short photoperiod did not have the same blossom-forming effect upon the two lots of trees: it induced blossoming of the low nitrogen trees but entirely prevented blossom bud formation on the high nitrogen trees. Reproduction is here obviously related to the chemical composition rather than to the treatment and is certainly not directly induced by either a short or a long day. With a well balanced nutrition, apple trees are apparently "long day" plants, that is, they form blossom buds during the long days of early summer.

The long day-low nitrogen trees could be classed if desired as high carbohydrate, under-vegetative trees, and the short day-high nitrogen trees as low carbohydrate, over-vegetative trees. In a similar way the short day-low nitrogen and long day-high nitrogen trees could be spoken of as ''balanced,'' fruitful trees. That is, fruitfulness accompanied a condition of intermediate amount of growth as well as nitrogen and carbohydrate content.

The behavior of the trees in a second season is shown by figs. 21 and 22. The removal of samples for analysis constituted a heavy pruning. The effect of this operation upon new growth was noticeably less pronounced than the influence of light environment. Attention is especially called to fig. 21, showing the growth made by trees practically without nitrogen in the nutrient for four or five seasons. Two years of short-day treatment has produced a slender type of growth with large thin leaves which is characteristic of over-vegetative trees. That is, reduced period of illumination gave a type of growth wholly comparable to making heavy applications of nitrate to trees with full illumination. The nitrogen-fed trees in the short day for two years produced partially chlorotic foliage and an extremely weak spindly growth.

TABLE VII
EFFECT OF PHOTOPERIOD UPON CHEMICAL COMPOSITION AND BLOSSOM BUD FORMATION IN DWARF APPLE TREES*

SAMPLE DAY
LENGTH
NITROGEN
NUTRIENT
MOISTURE TOTAL
N
STARCH REDUCING SUBSTANCES ACID
HYDRO-
LYZABLE
TOTAL
CARBO-
HYDRATE
RATIO TO N BLOSSOMS
FREE TOTAL STARCH CARBO-
HYDRATE
      Per cent. Per cent. Per cent. Per cent. Per cent.

Per cent.

Per cent.  

Per cent.

 
New growth Long Low1 46.1 0.70 8.08 2.43 5.25 7.01 20.34 11.5 29.1 0.0
Short Low 52.1 1.07 5.56 2.51 5.53 6.08 17.17 5.2 16.0 16.5
Long High 50.3 0.87 6.17 1.61 3.87 6.23 16.27 7.1 18.7 14.3
Short High 62.9 1.17 2.81 1.67 3.57 4.22 10.60 2.4 9.1 0.0
 
Two year
wood
Long Low 48.5 0.43 10.91 2.96 4.94 7.81 23.66 25.4 55.0  
Short Low 50.2 0.55 5.74 1.78 4.83 6.86 17.43 14.4 31.7  
Long High 48.0 0.38 5.37 1.42 2.20 3.98 11.55 14.1 30.4  
Short High 51.0 0.60 3.04 1.52 2.94 4.73 10.71 5.1 17.8  
*Analyses by N. MOGENDORFF.
1Traces of NO3 in tap water used. Other elements were supplied to the quartz sand used as soil.

Before proceeding to a further discussion of the chemical and growth data, it is well to inquire as to what significance the analyses have. What value should be placed upon the chemical data?

The reducing substances extracted in hot concentrated alcohol usually show little relation to vegetative condition. This might well be expected in view of its presence being dependent upon variations in light and temperature as well as upon utilization.

Starch is the one carbohydrate being analyzed for which does usually appear to be closely related to growth condition. Starch values are obviously high in some samples as has already been pointed out (4). Very probably boiling the sample to gelatinize the starch extracts some reducing substance. Consequently the range in variation of starch content is really larger than is indicated by analyses.

A discouraging fact is the failure to secure correlation between the acid hydrolyzable fraction and vegetative condition. Miss BRADBURY (1) has pointed out that this is probably because the wall thickenings which are present in apple tree tissues and function as reserves are not hydrolyzed in the course of standard analysis. Because of the marked relation between vegetative condition and anatomical development, the "hemicellulose" fraction should be expected to show a clear correlation to plant growth. Here the analyses clearly do not indicate as large differences as exist between samples.

There is a large discrepancy in the amount of material extracted and that recovered as reducing substances. This is not only large but it is also variable, ranging from approximately 30 to 60 per cent. in the samples reported upon in table IV. This difference can not be explained by clarification precipitates, as clarification increased the reducing power of more than a majority of the 20 samples compared, table VIII.

Discussion

From these and other observations it is hardly necessary to say that a mathematical ratio between materials as nitrogen and the carbohydrate fractions is only an indication of relations. It might almost be asked if careful histological and micro-chemical records of qualitative differences would not better indicate the course of metabolism than do the macro-analyses when present standard methods are followed. Better methods of analyzing for the usual substances are needed as well as methods for other substances than are now considered.

TABLE VIII
EFFECT OF CLARIFICATION UPON THE PERCENTAGES OF STARCH AND
ACID HYDROLYZABLE CONSTITUENTS OF SOME APPLE WOOD SAMPLES

SAMPLE STARCH ACID HYDROLYZABLE
CLARIFIED UNCLARIFIED CLARIFIED UNCLARIFIED
One year wood. Check 7.40 7.12 6.05 3.81
Shade 6.61 5.74 3.73 4.21
Girdled 9.92 10.71 7.65 5.62
Girdled and shade 8.88 9.99 4.31 4.85
Two year wood. Check 5.82 4.26    
Shade 5.07 4.28    
Girdled 8.29 6.18    
Girdled and shade 7.40 6.82    
Two year roots. Check 14.20 15.88 4.29 3.77
Shade 12.42 11.74 5.66 4.78
Girdled 15.98 12.85 5.54 5.30
Girdled and shade 4.02 4.02 6.32 5.88

Rather than suggesting that chemical data are of little importance because they show that different samples are not much different in composition, it seems to be a better interpretation to say that chemical data are often not of much value or significance because they represent only a part, too often small, of the real chemical differences existing between the samples, as is indicated by their marked differences in anatomical and growth character.

In addition to indications that blossom bud formation is related to a condition of "balanced" growth and composition, there is another significant situation which appeared in both series of trees. This is the relation of carbohydrate content or carbohydrate-nitrogen relation to vegetative elongation. This is shown particularly by the long growth of the short-day trees, especially those with a low nitrogen nutrient (fig. 21) and by the short growth of the girdled trees in the field.

How do very poorly vegetative trees become strongly vegetative by being placed in a short photoperiod? The change in percentage of nitrogen appears directly related to the fall in specific gravity due to a reduced carbohydrate content. With the respiration of carbohydrate material there is a corresponding liberation of available nitrogen, as NIGHTINGALE (3) has found in the tomato. If carbohydrate respiration is a liberator of nitrogen forms which make for increased growth then carbohydrate accumulation must have previously been a binder of those nitrogen forms. Does carbohydrate accumulation inhibit vegetative extension? Do accumulating reserves cheek growth as well as accumulate after growth in length is checked? That could be an interpretation of the reduced growth of girdled trees. Girdling so changes other factors that this evidence might be weak. The relation of amount of elongation to composition in the shade-and-girdling series does, however, appear suggestive. The probable, if not apparent, relation of carbohydrate accumulation to reduced growth has many practical applications in connection with such questions as dwarfness, period of growth, ''old age" in trees, partial etiolation, the rest period, as well as with blossom bud formation.

The "limiting factors" affecting growth may be within as well as without the plant. Clearly, relations rather than merely amounts of materials influence if they do not control vegetative response. The direct causes of growth responses should apparently be sought within the plant. At least, a fixed environment does not give a fixed response with plants of a different previous history. With agricultural perennials in a variable soil and climatic environment growth character should, then, be the basis for cultural treatment.

Summary

1. No separate environment had a consistent effect upon the composition, growth character or fruitfulness of young apple trees. For example, girdling checked growth and induced fruiting; shading of girdled trees neutralized the effects of girdling giving a growth situation comparable to ungirdled sun trees. Again, short-day trees without nitrogen grew and blossomed like long-day trees with nitrogen.

2. Growth character, including blossom bud formation, is primarily dependent upon internal composition and secondarily upon external environment. That is, of the five environmental conditions employed—sun, shade, girdling, photoperiod, nitrogen nutrient—none produced a specific growth situation.

3. The type of growth should be the condition used as a basis for deciding upon cultural practices.

4. Chemical analyses as developed at present provide only a limited and not too accurate method of measuring plant performance.

HORTICULTURAL DEPARTMENT,
UNIVERSITY OF WISCONSIN,
MADISON, WISCONSIN.

LITERATURE CITED

  1. BRADBURY, DOROTHY, and ROBERTS, R. H. The effect of acid hydrolysis upon a hemicellulose reserve in apple trees. Proc. Amer. Soc. Hort. Sci. 23: 298-299. 1926.
  2. KRAUS, E. J., and KRAYBILL, H. R. Vegetation and reproduction with special reference to the tomato. Oregon Agr. Exp. Sta. Bull. 149. 1918.
  3. NIGHTINGALE, G. T. The chemical composition of plants in relation to photoperiodic changes. Wisconsin Agr. Exp. Sta. Res. Bull. 74. 1927.
  4. ROBERTS, R. H. Apple physiology; growth, composition, and fruiting responses in apple trees. Wisconsin Agr. Exp. Sta. Res. Bull. 68. 1926.

EXPLANATION OF PLATES


PLATE II. Figs, 2-5. Upper epidermis of sun, shade, sun and girdled, and shade and girdled leaves. Camera diagrams by MARIAN DEATS ABEGG.
Figs. 6-9. Lower epidermis of sun, shade, sun and girdled, and shade and girdled leaves. Camera diagrams.
Figs. 10, 11. Camera diagrams of cross-section of leaves of sun and shade leaves.


PLATE III. Figs. 12-15. Camera diagrams of cross-section in pith of new wood: sun, shade, sun and girdled, and shade and girdled.
Figs. 16, 17. Pith cross-section in lower stems of sun and girdled and sun trees. Note utilization of starch and wall thickening reserves in 16.


PLATE IV. Figs. 18, 19. Cross-section diagram in new growth wood of shaded and sun and girdled trees.
Fig. 20. Trees with nitrogen. Left, long day; right, short day.


PLATE V. Fig. 21. Trees without nitrogen. Center, long day, two years; left, short day in 1925 and long in 1926; right, short day, two years.
Fig. 22. Trees with nitrogen. Right, short day in 1925, long day, 1926; left, short day, two years. Note the seasonal change in foliage character at left.