Agr. Exp. Sta. Research Bulletin No. 17 pp. 5-8 (April, 1915)
An Experimental Study of the Rest Period in Plants
W. L. Howard
PRELIMINARY EXPERIMENTS WITH SEEDS
The first of the rest period studies with seeds was begun in the spring of 1907. Several species of seeds, representing both wild and cultivated forms, growing in the vicinity of Columbia were collected and planted at different stages of maturity to find if they would grow at once.
The object in collecting immature seeds was to discover if possible just when the resting phase begins. It has been assumed that the rest does not set in until the seeds are mature. While this conclusion seems logical and appears to be warranted, there seems to be no experimental proof in support of such a conclusion. If seeds are able to grow before they are mature and not immediately afterward, it seems safe to conclude that the rest begins at the time of maturity. If seeds do not grow when immature and also do not grow immediately after maturity, such results would neither prove nor disprove when the rest period sets in. If they are able to grow before maturity and will not grow afterward, this would indicate that the rest period sets in with the conclusion of the ripening process. The process of germination in its last analysis is nothing more than the resumption of growth on the part of the embryo which has been dormant for a time. The use of the term "germination," then, in a way assumes that a seed is ripe or mature, and if it is ripe or mature the embryo has become dormant, sometimes even in an immature state. Presumably the embryo of a "ripe" seed is entirely mature or at rest. On the other hand it may be that immature seeds "germinate" because they are able to continue growing without becoming mature or entering into a state of rest.
The development of the seed from the fertilized egg cell in the ovule is a process of growth which involves not only the embryo itself which has to attain a certain size before it becomes dormant, but also the storage of reserve food either in the cotyledons, which are a part of the embryo, or in the form of endosperm which goes to make up the bulk of the "seed" or that part which is enclosed within the seed coat. When the reserve food is deposited in the cotyledon, or in the form of endosperm, the deposition continues to go on until the seed has reached a practical state of maturity. The external evidences of this stage are the usual ones which we are accustomed to associate with ripening seeds and which, of course, vary with the different species. In peas, for example, maturity is indicated by the hardening or drying of the pod and this, upon being opened, discloses that the seeds themselves are hardening and changing to a darker color.
In this experiment no provisions were made for morphological studies of the seeds so that the exact condition of the embryos at particular stages was not determined. Seeds were collected in two stages of immaturity. As nearly as could be determined, these were when "half ripe" and "nearly ripe." For various reasons it was possible to secure immature seeds, particularly in the half ripe stage, from but four species, and of the nearly ripe stage from only nine species. Fully ripe seeds were collected from twenty-two species. In all cases the seeds were planted in moist sand in the greenhouse.
TABLE 1. GERMINATION TEST WITH HALF RIPE SEEDS
|Hordeum sativum, Jessen||4||60.7|
|Pisum sativum, Linn||4||93|
|Raphanus sativum, Linn||4||63|
|Spinacia oleracea, Mill||12||23|
This preliminary test, while very incomplete in that it covers but little ground, is of interest because it shows that seeds of many annual plants are able to germinate when still quite immature, and the percentage that grow average as high as the growth from mature seeds planted at once after ripening. As a result of experience gained in the work, it is believed that where a species will germinate when immature, that it will also germinate with careful handling, immediately after ripening, or, in other words, it has no rest period. However, it should be clearly understood that this statement is advanced merely as an opinion, not as a fact that has been fully proven.
TABLE 2. GERMINATION TEST WITH NEARLY RIPE SEEDS
|Avena sativa, Linn||6||100|
|Hordeum sativum, Jessen||3.5||80|
|Phleum pratense, Linn||10||26|
|Pisum sativum, Linn||4||65|
|Raphanus sativum, Linn||2||63.3|
|Spinacia oleracea, Mill||7||16.6|
Ptelea trifoliaia, Linn, and Vitus Labrusca, Linn, failed to grow from slightly immature seeds.
TABLE 3. GERMINATION TEST WITH FULLY RIPE SEEDS
|Althaea rosea, Cav.||9||68|
|Cucumis sativus, Linn||5||75|
|Hordeum sativum, Jessen||3.5||51.6|
|Lactuca Scariola, Linn||17||2|
|Spinacia oleracea, Mill||7||73|
|Triticum vulgare, Vill.||5||90|
Aquilegia vulgaris, Linn., Cleome integrifolia, Torr & Gray, Coreopsis tinctoria, Nutt., Papaver sp., Panax quinquefolium, Linn., Rubus Phoenicolasius, Maxim., Rubus villosus, Ait., Rubus occidentalis, Linn., Rubus nigrobaccus, var sativus Bailey, and Vitis Labruesca, Linn., failed to germinate when planted immediately after ripening.
Table 3 shows that out of eighteen species tested for germination immediately following maturity, eight, or 44.4 per cent, grew. Since the list that grew immediately after maturity included annuals and woody and herbaceous perennials, it may be regarded as a fairly representative collection of plants, and even tho small, may be taken as a strong indication that almost half of the species of wild and cultivated seeds possess a rest period.
Two preliminary experiments in etherizing seeds were performed. First, nearly ripe seeds of barley (Hordeum sativum), and oats (Avena sativa), were exposed to ether fumes for twenty-four hours. Both grew in four days. The treatment hastened the germination by two days in the case of the oats, but only slightly in the case of the barley.
The second etherization test was with newly ripened seeds of fox-glove (Digitalis purpurea), larkspur (Delphinium sp.), dock (Rumex sp.), and tomato (Lycopersicum esculentum). The treatment was for twenty-four hours. All grew in from three to twelve days but germination was hastened but little, if at all.
These preliminary tests seem to indicate that treating with ether will not kill seeds even while in an immature state. Also it would seem that the treatment has more effect upon green seeds than upon ripe ones.