Chapter XI: Germination

Having considered the processes which lead up to the formation of seed, we may now investigate the life-history of a seed and its various forms.

Like fruits, seeds differ much in their outward shape. In size alone we find a great contrast between the dust-like seeds of the orchids and the huge seeds of the cocoa-nut-palm, while between those two extremes we may note every gradation of size. In other respects, also, the seed offers no less variety of form and covering than the fruit, such variations having relation to the particular mode of dispersion and germination. The outer skin or coat of a seed, called the _testa_, offers a very interesting field of study, and such seeds as the poppy and _silene_ with beautiful network, the _bignonia_ and _pinus_ with membraneous wings, the cotton-plant seed with long hairs, and the _collomia_ with hairs that are resolved into mucilage when wetted, are all worth special study. When a small portion of _collomia_ seed is moistened and placed in a microscope one may see the rapid change being effected; that which had been a hard dry atom suddenly throws out coils of gum, like watch springs, and a novice is led to ask, “Is the thing alive?” so full of motion does the object appear.

We may regard a seed under various aspects. As a special means of continuing the life of a plant, one of its modes of reproduction, as a special means of tiding a plant over a season that would be fatal to its life in its ordinary condition of leafage, in the seed we have the germ of the future plant, a reproduction of its parent. This germ or embryo is lethargic or hibernating like many animals which exist throughout the winter in a dormant condition, yet still continue to be living vital bodies waiting for some special influence to come into play, and ready to resume all the activity of a growing organism. The construction of a seed is simple; inside the coat or _testa_ we find the embryo with or without a special supply of albumen; if the seed is ex-albuminous, then we may expect to meet with thick, fleshy seed-leaves especially stored with this substance. The embryo contains all the essential parts of the plant, the root, stem, and leaves; the root in the seed state is called the radicle, and is that part of the embryo which usually points towards the micropyle; this radicle forms one end of the first shoot which comes out of a seed, the other end terminating in the stem or plumule. This first shoot is known by three names—axis, _tigellum_, or hypocotyle. The _tigellum_ in many plants gives rise to a special structure; thus in the cyclamen it forms the tuber, and the greater part of the “roots” of radishes and turnips is due to it. In other instances it is a mere collar forming a slightly thickened surface between the base of the cotyledon and the radicle. The _tigellum_ is in reality a centre of growth, as may easily be shown by cutting off an inch of the upper part of a well-grown carrot and placing the slice in a saucer of water; before long a crown of young leaves will spring up and will continue to grow and flourish as long as the plant food contained in the slice is sufficient to maintain the leafage. In botanical language we have thus been growing carrot leaves from this _tigellum_.

The embryo varies very much in the relative position of its parts. Thus the embryo of the reed-mace is straight in the _tigellum_ of the embedding albumen. In contrast to this is the curved embryo of the deadly nightshade and the spiral embryo of the hop.

The seeds of the orange often contain two embryos, which is rather a rare occurrence in the vegetable world. Before we can trace the future of these parts we must attain a clear idea of the change the seed undergoes when it germinates. In the whole of our studies our attention has been drawn to no process so deeply interesting and yet so mysterious as that of the breaking into life of the seed. There are three conditions that promote the process of germination: warmth, moisture, and air. When these three conditions are present and the seed is healthy, growth begins, and its first stage is the absorption by the seed of moisture; this, combined with warmth and the oxygen of the air, sets up a change in the contents of the seed. We have already seen that seeds are of a dry and starchy nature, and in this condition they are insoluble and unfit to be active plant food. The change that ensues results in this starchy matter being converted into sugar which is soluble; then the parts of the embryo begin to unfold, first the radicle and finally the plumule are developed. In this early stage these parts live entirely upon the contents of the seed, just as a young chick is developed and nourished upon the albumen of the egg.

The temperature requisite for germination varies according to the species; those of us who possess gardens know to our cost at what a low temperature such plants as chickweed, bittercress, groundsel, and some of the speedwells grow; as long as the thermometer is above freezing-point these troublesome weeds will make their appearance in our flower borders. Sach’s experiments on germination tend to show that wheat and barley begin to grow below five degrees centigrade, whilst French beans and maize germinate at nine degrees centigrade.

Some plants start into growth very quickly. Garden cress, vegetable marrows, and some grasses appear above ground a few days after they are sown, whilst other seeds, enclosed in a hard, woody seed-case, will require twelve months to germinate. This was the case with a seed taken out of a cedar cone brought from Mount Lebanon; I vainly watched for the young plant, and when a year had passed by the pot was thrown aside on a rubbish heap. Shortly after I was passing by and observed a fir-cotyledon growing on the heap, and this proved to be the long-desired young cedar-plant.

Seeds have the power to retain their vitality for years, especially those of the _Leguminosæ_, but I believe the stories of Egyptian mummy wheat germinating are scarcely to be believed. A good object-lesson upon this subject is furnished by a newly-made railway cutting; here we may always find growing upon the freshly-turned soil quite a crop of plants which have sprung from seeds that in the course of years have become embedded in the earth, it may be at so great a depth as to preclude the admission of air or prevent one of the necessary conditions of germination. When, however, the underlayer of soil is brought to the surface and exposed to light, air, and moisture, the seeds are able to grow.

To this we owe the richness of our railway-bank flora, and many a rare plant may be discovered there which cannot be found elsewhere in the neighbourhood. We will now in imagination conduct a few simple experiments that we may learn something of the behaviour of seeds during their early stages of growth. Each seed that we thus study may be regarded by us as a type of many others. First, then, we will sow, in a few pots, about a dozen broad beans; before doing so we may notice on the seed the black stripe or ridge known as the _hilum_; this is the scar showing where the seed was attached to the pod, and at one end of it is the micropyle (_small gate_). If we remove the skin of the seed we shall observe the two fleshy cotyledons or seed leaves, a tiny point which is the rudimentary root, and, lying close to the inner face of the cotyledon, the slightly curved plumule. After the beans had been sown a few days and carefully watered, we may take up two or three for examination. At first we may only see the radicle just emerging from the little hole at the end of the _hilum_, but if we wait, say, eight or nine days, we shall get a further development.

Before digging up our seed we will see if any others are peeping through the soil. Yes, here is one, just an arched kind of shoot, no leaves, only the bow of the arch pushing up the particles of the soil, so that the point of the shoot is clearly still below the ground. Now, taking up a seed we notice that the radicle has penetrated some way down into the soil, and with a pocket lens we are able to see a little higher than the tip of the root quite a crop of delicate little root-hairs. The cotyledons are still enclosed in the tough skin, but the upward growth of the _tigellum_ is acting on them like a lever, and we can now plainly see that it is this _tigellum_ that, by its upward growth, is penetrating the soil, and in so doing is drawing the cotyledons from the seed coat. All this time the delicate plumule is kept out of danger by the arched shape of the _tigellum_ and the folding of the cotyledons. Leaving our seeds for a day or two longer we find a further change. The plumule has been carried up beyond the soil-level and has begun to expand into leafage. It is interesting to note how the curved _tigellum_, pushing through the soil first, effectually guards the plumule from injury arising from contact with rough particles of earth; the cotyledons remain just below the soil-level and we see that the _tigellum_ is thickening and forming a distinct connecting branch between the new shoots and the fleshy seed leaves; these latter are full of plant food, and the plumule is supplied from this storehouse of nutriment until the first leaves are formed and are able to decompose carbon-dioxide for the nourishment of the plantlet. The seed-leaves in this case do not perform this function, but act simply as storehouses.

Our next seed example will be the familiar mustard plant. These we may sow in two lots, the first we only need to sprinkle upon some fine soil and the second may be sown in a shallow drill and covered with fine earth.

The first sowing will quickly germinate, and the movement of the radicle which pushes out of the micropyle may be understood by reference to the appended diagram. In it we see the white thread-like radicle emerging from the seed coat; it turns very quickly towards the ground and pushes directly into the soil. Here I must direct my readers’ attention to one of those minute arrangements which, though apparently insignificant enough if we fail to study the context, is really an evidence of the infinite perfection, care, and wisdom of the Creator in even such a tiny detail as the springing up of a mustard seed. As the seed lies upon the ground, the lengthening radicle, while it penetrates the ground, has a tendency to force the seed into the air (as shown in the illustration), and were it allowed to do so the seedling would soon shrivel up and die. This catastrophe is, however, averted by the development upon the radicle of quite a crop of fine white root-hairs; these adhere closely to the minute particles of the soil, and are thus enabled to counteract the force exerted by the tip of the radicle; the latter pushes through the ground without uplifting the seed. This action can be watched and the growth of the root-hairs observed by means of a pocket lens and by the exercise of that virtue, most necessary for all young naturalists—patience.

Returning to the seeds that were sown under the soil, we find they have germinated; the radicle is pushing downwards, and just above the soil-level we may see the short curved _tigellum_. This very quickly straightens itself, and then we observe that the cotyledons have been drawn out of the seed-coats and are displayed as two green leaves, which in a few days will be an inch or two above the ground, owing to the growth of the _tigellum_. Here we get quite a departure from the bean seed, whose cotyledons were _hypogean_ (under the earth), those of the mustard being _epigean_ (upon the earth). There is also another point of difference; the mustard cotyledons are green, they contain chlorophyll corpuscles, have stomates, and so can perform all the functions of the normal green leaf; thus they help at once to feed the young plantlet by decomposing the carbon-dioxide of the air and forming starch, whilst in contrast to this we learnt that the seed-leaves of the bean were storehouses only. We are now sufficiently acquainted with the functions of the seed to be able to appreciate the variations of the _testa_, or seed-coat. In numerous instances the spines, prickles, hairs, and other growths on the surface have, in addition to their use in dispersing the seed, an essential purpose in holding the seed in its rightful position. We will take cress as our next example, since it may be regarded as a type of all smooth seeds. Cress seed remains intact until water comes in contact with it; then it becomes slimy by the liberation of a mucilaginous cement from the outer coat layer; this is, of course, highly adhesive, and thus the seeds are fixed firmly into the soil.

Another example is that of the little epiphyte (mentioned in our first chapter), _Tillandsia usneoides_, or old man’s beard. When the seeds leave the capsule they are furnished with silky hairs, which enable the tiny little structures to float through the air; they soon come in contact with the bark of trees, and then the little hairs cling to the rough surface. In this position the seeds germinate, and are held firmly in their place by the tightly-clasping silken strands.

Hardly any pursuit is more delightful than the collecting and drying of seedling trees; a ramble through the woods in early summer will reveal many specimens under or near the outskirts of the foliage. Under the beeches we shall soon light upon the nuts of last year coming up through the moist, rotting soil, in the form of two broad, green seed-leaves. As they often retain the dry, three-cornered seed-husk upon them, we can easily see that they are young beeches; otherwise, the cotyledon leaves being so unlike the perfect form, it might be rather difficult to distinguish the species. These seedlings have germinated somewhat like the bean seed, the radicle has grown downward, and the curved _tigellum_, pushing upwards, has drawn the cotyledons out of the seed-coat. We may notice with surprise through how small an aperture the cotyledons have been pushed, and still they are uninjured, a fact that is due to their being folded up like a fan in the seed-husk. As soon as the _tigellum_ reaches light and air it straightens out, and the flat seed leaves, which are at first of the palest green, soon deepen in colour, and are working away preparing food for the growth of the young plumule which springs up from between the cotyledons, crowned with two perfect young beech-leaves. This is all the baby-tree can do the first year. We can distinguish the second-year seedlings by their woody stem, brown leaf-scales, and silken-fringed young beech-leaves.

We shall not find cotyledons on the young oak, horse-chestnut, or sweet-chestnut seedlings, because these remain normally below the ground (hypogean), forming a storehouse of nutriment for the young tree. It is interesting to watch the growth of an acorn when placed in damp moss in a saucer. After a few weeks the acorn will have absorbed water, and the leathery seed-coat will burst at the pointed end; through this rent the radicle will protrude, fibres will be found growing upon the root, the _tigellum_ is thick, and just where the stalks of the cotyledons are joined to it the plumule emerges as from a sheath. The plumule is in no hurry to develop leaves; its first growth is provided for by the rich supply of food within the acorn. If, however, we look carefully at its little stem, we shall observe upon its surface a few scattered scales, each with a rudimentary bud in its axil. When the shoot has attained a height of three or four inches it develops its first green leaf, and by the end of its first summer about six will have been formed. A collection of these seedling trees, carefully dried[24] and neatly arranged in a blank book, with the English and Latin names to each, a note of the age of the seedling, the spot where it was obtained, and the date, will in time form a pleasant memento of forest rambles, and, probably, may lead to further studies of a similar kind.

[24] They merely need to be placed between sheets of blotting paper, which should be dried daily and kept in a press or under a weight for a few days until the specimens are fit to be placed in a book.

To make the collection complete there should be some seedlings of the other great division of plants, namely, the plants with one seed-leaf (monocotyledons). A few date-stones will supply these specimens; they should be sown in moist earth and placed either in a greenhouse or on a sunny window-ledge, where their growth can be watched.

Their germination is quite different from that of the other seeds we have described, and if a number of seeds are sown the different stages can be seen as in the accompanying figure.

One long cotyledon is pushed out from the seed, the free end is like a sheath. The part nearest the seed forms a structure resembling a rolled-up stalk; from the former roots are developed, whilst from the rolled-up stalk or sheath grows the next formed leaf, and each successive leaf is sheathed like its predecessor. This arrangement can be well seen in young growing grasses which can be taken to pieces and examined. I shall conclude this chapter with a brief reference to the spores or so-called seeds of ferns and mosses.

These are essentially different from the seeds that have formed our study in the earlier part of this chapter, they do not contain an embryo. Let us first notice fern-spores, which we shall find in abundance at the back of maiden-hair and other fern fronds; they are contained in little brown patches known as spore cases (_sporangium_, from _spora_, a spore, and _aggeion_, a vessel). If we collect some of these and sow them on some very fine damp earth, keeping it at the same time shaded and warm, the spores will soon germinate. We shall not find a radicle this time as the result of growth, but in its stead a flat expansion of green tissue (prothallium, Gr. _protos_, first, _thallos_, a branch) growing upon the earth like an exceedingly delicate leaf. From the underside of this green film a few very fine root-like hairs (rhizoids, Gr. _rhiza_, a root) are developed; very soon with a microscope we shall be able to discern upon the surface of this structure a few little projections. In one of these is developed a flask-shaped mass of cells (archegonium, Gr. _archegonos_, first of a race), in the other (antheridium, diminutive of Gr. _anthera_, an anther) some minute bodies (antherozoides, Gr. _anthera_ and _zooid_, a minute life) with tails; these escape from the covering and wriggle about very much like tiny animalcules until finally they come into contact with the flask-shaped opening before mentioned. These tailed structures are something like pollen grains in their function, only they differ from pollen grains, which are passive, by being endowed with the power of motion; the result of their fusion with the flask-like body is to fertilise the germ cell (oospore, Gr. _oon_, an egg) in that structure, and from the germ cell so fertilised is developed an embryo from which at once springs the young fern plant. The first leaf grows from the upper part of the embryo and from the lower part is developed the “foot,” a little connecting-link between the green prothallus and the baby fern which serves to nurse the little plant until two or more leaves have been produced; the roots also grow from the same part of the embryo. I imagine that fern spores could be grown and watched through all their various stages even by those of my readers who dwell in towns, as a bell glass would maintain the requisite dampness and shelter the young ferns from smoky air.

Lastly I will describe an even simpler form of spore development. At any season of the year we may find the capsule fruit of mosses (Calyptra, Gr. _Kaluptra_, a veil), a very common one being the hair moss (Polytrichum, Gr. _Polutrichos_, having much hair), borne upon long wiry stalks. Inside the capsule we shall find a large quantity of small greenish bodies; these are the spores, which of course fall out when the spore-case is blown by the wind, and being light are easily carried away and at length find a resting-place in some damp nook or shady bank. In such a place they find the conditions necessary for their germination, which is not unlike the same process in other seeds and spores we have studied. The result is very simple. A fine, silky, thread-like body (protonema, Gr. _protos_, first, and _nema_, a thread) is developed; when this has attained a fair size, a little moss plant begins to grow upon its surface exactly as we see a bud grow upon a tree-branch, and it is upon this moss plant that the organs of reproduction are produced. We have now come to the end of our study of seeds.

An endless source of interest to the student of nature is opened up to view by carefully observing the beginning of all vegetable life, and the seed or spore of the commonest weed or fern will teach us lessons that should ever make us mindful of the wonderful mystery of life and its genesis.

Objects to collect and examine:—Poppy, silene, and collomia seeds. Examine _tigellum_ of cyclamen, radish, and carrot. Sow broad beans, mustard, and cress seed. Collect seedling trees. Sow date-stones. Examine fern and moss spores.

CHAPTER XII

_THE PHYSIOLOGY OF PLANTS_

“Lo! on each seed, within its tender rind,
Life’s golden threads in endless circles wind;
Maze within maze the lucid webs are roll’d,
And, as they burst, the living flames unfold.”

+Erasmus Darwin+, _The Botanic Garden_.