2. The yellow lupine.—In the same way observe how this seed grows. Do the cotyledons shift their position or change in colour? Do they become leaf-like? How do they differ from later-formed leaves? What becomes of them at last? What becomes of the seed-coat?
3. The mustard seed.—Notice that, soon after the radicle has come out of the seed-coat, a sort of hump forms at its upper end, and at length the cotyledons are pulled out of the seed-coat and turn up towards the light. What is their colour? Observe that the two cotyledons are soon raised on the end of a little stalk. Like the cotyledons of the yellow lupine they are plainly leaves. Notice their shape. Are they of equal size? Why not? When they are about three inches above the seed-coat gently separate them and notice the little bud between them. Draw the seedling. How large can you get a mustard seedling to grow on damp flannel? Plant a few mustard seeds on earth, and notice the difference between the shape of the cotyledons or seed-leaves and that of the leaves which appear later. What becomes of the cotyledons?
4. The vegetable marrow seed.—Make similar observations upon the vegetable marrow seeds, noticing particularly whether the cotyledons remain in their original positions and shrink up as the plant increases in size, or whether they are pulled out of the seed-coat by the elongating stem, and become green and leafy. How does the plant hold down its seed-coat whilst it pulls out its cotyledons?
5. The sycamore seed.—From what you have seen of the cotyledons of the sycamore seed, will you expect them to behave like those of the mustard seed, or like those of the pea and bean? Even in the seed they are green, and plainly leaves. How do they escape from the seed-coat? What is their shape? Do they come out before or after the radicle? Do they get any larger as the stem grows? How large can you get a sycamore seedling to grow in damp sawdust? As large as a seedling of pea or bean? Plant some sycamore seeds on earth and compare the shape of the cotyledons with that of the next-formed leaves. How soon do the “true” leaves appear after the cotyledons have escaped from the seed? Do any “true” leaves grow on the plants in sawdust? What becomes of the cotyledons at last?
The embryo.—The plumule, radicle, and cotyledons, which have now been seen in the seed, form the embryo of the plant. The adult plant will be wholly formed by the growth and development of these parts, and we must now follow carefully the changes which take place when the seed germinates, and try to find out what becomes of each part. It is better to put the seeds at first in damp sawdust rather than in earth, as the young roots can then be more readily cleaned and observed. With small seeds the early stages of growth are better seen if damp flannel is used.
Germination.—Under the influence of moisture and warmth the embryo in the seed begins to swell and unfold its parts. The radicle makes its appearance first ([Fig. 5]), breaking through the seed-coat at the micropyle; it is the young root. The radicle always grows downwards, that is, toward the centre of the earth. If the seed lies in such a position that the micropyle is directed upwards, the point of the radicle turns over and grows downwards as soon as it escapes from the seed-coat. As the young root becomes longer and thicker ([Fig. 6]) the seed-coat opens more and more, showing the cotyledons beneath, and these, too, are gradually forced apart.
| Fig. 5.—An early stage in the germination of a Broad-Bean seed. R, radicle; s.c., seed-coat. (× ⅔.) | Fig. 6.—A slightly later stage in the germination of a Broad-Bean seed. cot., cotyledon; pl, plumule; R, radicle; s.c., seed-coat. (× ⅔.) |