This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
To watch the growth of various seeds is an extremely interesting study, and one of which I am very fond. I often grow some Peas and Beans on a very damp cloth or wet moss : the latter is best, as it retains moisture much longer than the former. It must not, of course, be supposed that they will continue to grow very long, as they will simply do so until the radicle and plumule are about a quarter to half an inch long. A better plan still is to grow them in a light soil and examine them every week, when you cannot fail to notice the elongation of the radicle, etc.
To give the reader a better idea of the inner structure of the cotyledons, and also bud of the future plant, here is a little sketch of an embryo of the Pea, minus its skin, and opened carefully, c c represents the cotyledons or seed-lobes; t, common axis, whence both radicle and plumule proceed, which is united by a short petiole; p, plumule; r, radicle; and d, a small aperture in which rested the bud.
We may reasonably infer that, before the radicle is capable of obtaining its own food, it is supplied to it from the cotyledons; the radicle, as it increases in size, sinks into the earth, assumes the form of a root, and becomes not only its own food-supplier, but that of the future plant. Even at this period, after the radicle has become a perfect root, the plant, as Sennebier ascertained by experiment, ceases to germinate if the cotyledons be cut off. They are still, then, absolutely necessary for the vegetation of the plant. The cotyledons now assume the appearance of leaves, and appear above the ground, forming what are called cotyledon-leaves of the plant. After this the plumule gradually increases in size, rises out of the earth, and expands itself into branches and leaves. The cotyledon-leaves soon after this drop off and decay.
As it does not appear that there is any communication between the cotyledons and the plumule, we must infer that the nourishment that the plumule obtains must pass into it from the radicle; and accordingly, we see that the plumule does not begin to vegetate until some time after the radicle has made a little progress. Since the plant ceases to vegetate, even after the radicle has been converted into a root, if the cotyledons be removed before the plumule is developed, it follows that the radicle is insufficient of itself to carry on the processes of vegetation, and that the cotyledons still continue to perform a part - and that is, they prepare food for the nourishment of the plant. When the young plant assumes the form of cotyledon-leaves, it is very evident that the nourishment, which was originally laid up in them for the support of the embryo plant, is exhausted, yet they still continue as necessary as ever. They must therefore receive the nourishment which is imbibed by the roots : they must produce some changes on it - render it suitable for the purposes of vegetation - and then send it back again to be transmitted to the plumule.
After a plumule has acquired a certain size, which must at least be a line, if the cotyledons be cut off, the plant, both Bonnet and Sennebier ascertained by experiments, does not cease to vegetate, but it continues always a mere pigmy - its size when compared with that of a plant whose cotyledons are allowed to remain, being only as 2 to 7. When the plumule has expanded completely into leaves, the cotyledons may be removed without injuring the plant, and they very soon die of themselves. It appears, then, that this new office of the cotyledons is afterwards performed by that part of the plant which is above ground. For a fuller explanation, and various modes of germination, see the 'Gardener' of September 1880, page 414.
The embryo is usually solitary in the seed, but occasionally there are two or several.
When several embryos are produced within a single seed, it sometimes happens that two of these embryos grow together, in which case a production analogous to animal dicephalous monsters is formed.
The number of cotyledons varies from one to several; commonly two, in which case they are mostly opposite : Ex. Pea, Bean, etc.
W. Roberts. (To be continued).