Seeds And Seedlings
( Originally Published 1915 )
Vitality and longevity of seeds. Conditions favouring germination. Seedlings. Variation. Natural and artificial selection. Sports or mutations. Hybrids and the laws of heredity.
Ripe seeds are surrounded by a hard seed coat which is more or less impervious and prevents complete drying up of the embryonic plant within the seed. The more resistant the coat the longer the seed can preserve its vitality.
The seeds of some plants germinate almost immediately they are mature, but most of them are adapted for and require a period of rest. During that time it is necessary to keep them dry and fairly cool. Under such conditions they can preserve their power of germination for some years, though a certain number even of resistant seeds will die. It is also more difficult to germinate old seeds, owing probably to the drying of the seed coat as well as to changes which have taken place in the living cells of the seed. When kept dry little alteration takes place internally, and the seed can remain dormant, practically no loss of matter taking place by respiration. For what length of time this suspension of animation can last is not definitely ascertained, but we may say with certainty that we have no proof that seeds which have lain dormant for a thousand years or more, like those which have been taken from Egyptian mummy cases, can be germinated. That so-called mummy wheat and mummy peas have been originally obtained from Egyptian mummies must be regarded as purely legendary. Accurate investigations which have been made regarding the longevity of seeds* have proved that so far a hundred years may be taken as the longest period for which seeds have been known to retain their power of germination. The seeds which enjoy such prolonged vitality belong to a restricted number of Natural Orders of which the Pea Family is one, remarkable like the others for the very resistant coat or testa with which its seeds are covered.
After the resting period, seeds when placed in favourable conditions commence to germinate. Moisture and a warm temperature favour germination. Water is absorbed by the seed coat as a whole, or may be taken in at certain points. In some small smooth seeds the outer layers of the seed coat are mucilaginous, and when wetted swell up considerably and become slimy. This is the case with the seeds of the flax plant (linseed) and with those of the cress. The probable reason for this special provision is to cause the seeds to become fixed in the soil so that the seed leaves can be more readily withdrawn from the seeds. There are generally present on the inside of the seed coat certain layers of cells which swell up rapidly when the seeds absorb water, and probably aid in the splitting of the seed coat, thus enabling the embryo to be gradually withdrawn from the seed. Warmth, the other factor essential to germination, is required for certain important chemical changes which need to take place before germination can be effected. The food material stored, in the seed is largely of a solid nature and requires to undergo trans-formation so that it can be dissolved in the cell-sap and can. be conducted to the growing root tip and to the developing leaves. By certain fermentative changes starch is converted into sugar, ,oil and organic nitrogenous coin-pounds are broken up and pass from cell to cell. In some cases as in peas and beans the food material is stored in the two fleshy seed-leaves or cotyledons, while in other seeds like those of the melon, the onion, and in all our cereals it is found in cells outside the young seedling and requires to be absorbed by the latter before it can be made use of at the growing points. The re-awakening of the vital processes indicated by these internal changes is marked by the commencement of respiration, that indispensable accompaniment of all life.
Let us now pass on to consider the nature of the seedlings themselves. It was pointed out previously that Darwin had shown the beneficial effect of cross-pollination in the more vigorous development of the offspring. Another important feature of seed reproduction as compared with vegetative reproduction, particularly if the seeds are the result of cross-fertilisation is the occurrence of a considerable amount of variation. The more dissimilar the parents the more varied are the offspring, and the greater therefore the scope for the play of natural or artificial selection.
The term variation has been used for two very different phenomena noticeable when examining a large number of plants or animals of the same species or kind. It is well ]sown that the offspring of any two parents all have some different individual characteristics, and a close observation of a number of seedling plants will show us that though they all have a general resemblance, we find that they differ slightly one from another in size, in the shape and texture of their leaves, and when they grow up in colour and conformation of their flowers. Indeed, if we had carefully examined the seeds from which they have grown we should have found that the latter showed already a considerable range of variation in shape and size, and possibly also in colour. Such slight individual variations are always found to fluctuate around a mean or average which we may look upon as the general character of the species or race. It is these slight individual variations which Darwin regarded as so important in the evolution of new forms, those least suited to the particular conditions of life gradually dying out and leaving those which were fit to survive. Natural selection operating in this way was thought to have produced the innumerable forms which we know as natural species. In the same way man, by making a choice of the plants most suitable to his purposes, has by artificial selection produced the strains and varieties now cultivated.
Recently experiments have been carried ,out in which the seeds of certain plants have been carefully graded and the largest sown separately with a view to ascertaining whether by such constant selection the seeds could be in-definitely increased in size.' This was, however, not found to be possible, for though the average size of the seeds was at first considerably raised, a limit was reached beyond which it was impossible to increase the size of the grains. By this process of selection the investigator had probably succeeded in isolating a pure strain of the particular variety with which he was experimenting, characterised by a larger seed than the sample with which he started, which was, no doubt, a mixture of races, some producing smaller and some larger seeds. But once a pure line had been obtained, though it showed slight fluctuations, yet it could not be improved by selection of the extremes of these fluctuating variations.
We know, however, of other variations which arise from time to time in all species of plants, and which are very different from the fluctuations as we may term those described above. Sometimes we come across new forms which differ considerably from the normal type in one or more characters, as, for example, the cut-leaved varieties of so many plants. In this case we do not find a series of forms intermediate between the cut-leaved individual and those with normal foliage. This second type of variation which arises suddenly Darwin called a sport, and he considered it to be of comparatively little importance in the evolution of plants, as its very infrequent occurrence would cause it to disappear in nature by constant inter-crossing with the more numerous normal forms. By artificial selection, however, man can perpetuate and establish these sports, as has obviously been the case with many of those forms which took the fancy of the horticulturist. It is to this kind of variation that we owe so many of our interesting and peculiar forms of cultivated plants. Recently a Dutch botanist, De Vries, has endeavoured to show that this second kind of variation, which he calls mutation, to distinguish it from the former or fluctuating variation, is of more frequent occurrence than had been supposed by Darwin. He observed that a certain large group of plants of Evening Primrose which had established itself in a wild condition in Holland showed a very considerable amount: of mutation, and his experiments and other observations led him to the conclusion that at certain periods, possibly owing to changed environment, plants passed into a phase of mutation, during which numerous new sub-species or races might arise. Interesting and important as De Vries' experiments are, his case cannot be considered proved until we know more about the previous history of the plants which show such considerable mutation. At present we do not know whether the Evening Primrose which he investigated was a pure race or of hybrid origin.
To horticulturists a knowledge of hybrids is of the greatest importance, a vast number of new and interesting forms being continually produced through hybridisation. At one time it was thought that hybrids were invariably intermediate between the two parental forms, and that they were generally sterile, and could therefore not be reproduced by seed. Though this is sometimes the case, particularly where the parents are of different species, it is by no means the rule, and certainly not in the case of hybrids between two different varieties. Our exact know-ledge of hybrids, dates from the careful experiments made in the latter half of the nineteenth century by Gregor Mendel, of Brunn. Unfortunately, his important investigations did not become generally known until the beginning of the present century. Mendel's first observations were made on various varieties of the garden pea, and he obtained the striking result that in crossing two different strains the offspring were not of immediate type, but generally inherited the characters of one of the parents in their entirety. Thus in crossing varieties, of which one had round and. smooth and the other wrinkled seeds, he obtained seeds all of which were round. He therefore considered this to be a dominant character, while he called wrinkledness recessive. But when the flowers of the hybrid plant were subsequently fertilised with their own pollen, the seeds they produced were not all round, some of them were wrinkled like those of one of the grand parents. On carefully counting the number of these recessive types, he found that one in four had reverted to the wrinkled type. Moreover, he found that of the round seeds some were of pure type, and when further cultivated always, produced round seeds, while others were of hybrid nature and these always produced reversions to the parents which had been originally crossed.
He was able, finally, to demonstrate that of the off-spring of every hybrid when self-fertilised one quarter reverted completely to the female parent, one quarter to the male parent, while half of the offspring remained of hybrid nature. These resembled the parent, which possessed the dominant character but preserved the recessive character in a dormant or latent form as was shown by its reappearance in a subsequent generation. The accurate numerical results, which Mendel obtained, can be readily explained on mathematical grounds by what we know of the various combinations which are possible between egg-cells and pollen-grains when. differing in two characters. Of course, if the parental forms differ in several characters the possible combinations are much more numerous and the numerical chances of complete reversion to the parent forms is much smaller. On the other hand, while we obtain a large number of hybrid forms which will show reversions to the original parents, we shall find some new combinations which are pure forms and therefore breed true. These we can isolate from the rest by the rejection of all strains which show reversion, and thus we can obtain new and permanent varieties. This no doubt: has been the means adopted by plant breeders in the past from practical experience, but thanks to Mendel and those who have followed up the path shown us by his investigations, a scientific basis has been laid to the practice of hybridisation.
In the course of these scientific enquiries some remarkable facts have come to light. It has, for instance, been discovered in crossing a white and a yellow variety of the Marvel of Peru (Mirabilis) that the hybrid produced was not intermediate in character, that is, of pale yellow colour, neither was it like either of the parents, but of a pink colour and, marked with red stripes. The white form must evidently have possessed some chemical factor which changed the yellow colour into red, while the character producing striping, which could not show itself in the white form, became visible in the hybrid by reason of the coloured sap. The offspring produced by self-fertilisation of this hybrid were of twelve different kinds: five with different shades or striping in yellow, five corresponding forms in red, and two white forms, which, though resembling each other externally differed in constitution as could be seen from t heir progeny.
A somewhat similar and equally remarkable result was obtained by crossing two white Sweet Peas belonging to the variety, Emily Henderson, the offspring having a partly-coloured flower, red with blue keel, probably very like the ancestral form. Of the two white forms evidently each one contained a special factor, which combined with that of the other white form caused the formation of a coloured sap. The offspring of the coloured hybrid were mostly coloured but in different degrees, while white forms similar to the two original parents were also produced, the two colour-producing factors separating out in these particular descendants.
Though in a comparatively short space of time great advances have been made in our knowledge ,of the laws of hybridisation and heredity, much remains still to be learnt in this important branch of scientific knowledge and the cooperation, of the plant breeder with the scientific investigator is much to be desired. The success already attained in this country is of good augury for the future.