by Grant Allen
Dicotyledonous is a very ugly word, and I shall not stop now to explain it from the top of a five-barred gate. It must suffice if I tell you confidentially that the little plant we have ideally reconstructed was the first ancestor of almost all the forest trees, and of all the best known English herbs and flowers; but not of the lilies, the grasses, and the cereal kinds, which belong to the opposite or monocotyledonous division of flowering plants. When this sprig of goose-grass first appeared above the ground, it probably represented that typical ancestor almost to the life; for it had then only the two rounded leaves you see at its base, and none of these six-rowed upper whorls, which are so strikingly different from them. Now, how did the upper whorls get there? Why, of course they grew, you say. Yes, no doubt, but what made them grow? Well, the first pair of leaves grew out of the seed, where the mother plant had laid by a little store of albumen on purpose to feed them, exactly as a reserve of food materials is laid by in the egg of a hen to feed the growing chick. Under the influence of heat and moisture the seed began to germinate, as we call it — that is to say, oxygen began to combine with its food stuffs, and motion or sprouting was the natural result. This motion takes in each plant a determinate course, dependent upon the intimate molecular structure of the seed itself; and so each seed reproduces a plant exactly like the parent, bar those small individual variations which are the ultimate basis of new species — the groundwork upon which natural selection incessantly works. In the case of this goose-grass seed the first thing to appear was the pair of little oval leaves; and, as the small store of albumen laid by in the seed was all used up in producing them, they had to set to work at once manufacturing new organic material for the further development of the plant. Luckily they happened to grow in a position where the sunlight could fall upon them — a good many seedlings are more unfortunate, and so starve to death at the very outset of their careers — and by the aid of the light they immediately began decomposing the carbonic acid of the air and laying by starch for the use of the younger generation of leaves. At the same time the vigorous young sap carried these fresh materials of growth into the tiny sprouting bud which lay between them, and rapidly unfolded it into such a shoot as you see now before you, with level whorls of quite differently shaped and highly developed leaves, disposed in rows of six or eight around the stem.
Observe that the adult type of leaf appears here suddenly and as it were by a leap. If we could reconstruct the whole past history of the goose-grass, we should doubtless find that each change in its foliage took place very gradually, by a thousand minute intermediate stages. Indeed, many of these stages still survive for us among allied plants. But the impulsive goose-grass itself clears the whole distance between the primitive ancestor and its own advanced type at a single bound. The intermediate stages are all suppressed. This is not always the case: there are many plants which begin with a simple type of leaf, and gradually progress to a complex one by many small steps; just as the tadpole grows slowly to be a frog by budding out first one pair of legs and then another, and next losing his tail and his gills, and finally emerging on dry land a full-fledged amphibian. The goose-grass, however, rather resembles the butterfly, which passes at once from the creeping caterpillar to the complete winged form, all the intermediate stages being compressed into the short chrysalis period; only our plant has not even a chrysalis shape to pass through. It is in reality a very advanced and specially developed type — the analogue, if not of man among the animals, at least of a highly respectable chimpanzee or intelligent gorilla — and so it has learnt at last to pass straight from its embryo state as a two-leaved plantlet to its typical adult form as a trailing, whorled, and prickly creeper.
And now let us next look at this adult form itself. Here I have cut a little bit of it for you with my penknife, and, if you like, I will lend you my pocket lens to magnify it slightly. The fragment I have cut for you consists of a single half-inch of the stem, with one whorl of six long pointed leaves. You will observe, first, that the stem is quadrangular, not round; secondly, that the leaves are lance-shaped, not oval; and thirdly, that both stem and leaves are edged with little sharp curved prickles, pointing backward the opposite way to the general growth of the plant. Let us try to find out what is the origin and meaning of these three marked peculiarities.
Fig. 24. — Stem of Cleavers.
To do so rightly we must begin by considering the near relations of the goose-grass. In a systematic botanical classification our plant is ranked as one of the stellate tribe, a subdivision of the great family of the Rubiaceæ, or madder kind. Now, the stellates are so called because of their little star-shaped flowers, and they are all characterised by two of these goose-grass peculiarities — namely, the square stems and the whorled leaves — while the third point, the possession of recurved prickles on the angles of the stalk and the edges of the leaves, is a special personal habit of the goose-grass species itself, with one or two more of its near relations. It will be best for us, therefore, to ask first what is the origin and meaning of the characteristics which our plant shares with all its tribe, and afterwards to pass on to those which are quite confined to its own little minor group of highly evolved species.
What, then, is the use to the goose-grass of these small, narrow, thickly whorled leaves? Why are they not all and always large, flat, and oval, like the two seed leaves? The answer must be sought in the common habits of all the stellate tribe. They are without exception small, creeping, weedy plants, which grow among the dense and matted vegetation of hedgerows, banks, heaths, thickets, and other very tangled places. Now, plants which live in such situations must necessarily have small or minutely subdivided leaves, like those of wild chervil, fool’s parsley, herb-Robert, and fumitory. The reason for this is clear enough. Leaves depend for their growth upon air and sunlight: they must be supplied with carbonic acid to assimilate, and solar rays to turn off the oxygen and build up the carbon into their system. In open fields or bare spaces, big leaves like burdock, or rhubarb, or coltsfoot can find food and space; but where carbonic acid is scarce, and light is intercepted by neighbouring plants, all the leaves must needs be fine and divided into almost thread-like segments. The competition for the carbon under such circumstances is exceedingly fierce. For example, in water only very small quantities of gas are dissolved, so that all submerged water-plants have extremely thin waving filaments instead of flat blades; and one such plant, the water-crowfoot, has even two types of foliage on the same stem — submerged leaves of this lace-like character, together with large, expanded, floating leaves which loll upon the surface something like those of the water-lily. In the same way hedgerow weeds, which jostle thickly against one another, have a constant hard struggle for the carbon and the sunshine, and grow out accordingly into numerous small subdivided leaflets, often split up time after time into segments and sub-segments of the most intricate sort. I do not mean, of course, that each individual leaf has its shape wholly determined for it by the amount of sun and air which it in particular happens to obtain, but that each species has slowly acquired by natural selection the kind of leaf which best fitted its peculiar habitat. Those plants survive whose foliage adapts them to live in the circumstances where it has pleased nature to place them, and those plants die out without descendants whose constitution fails in any respect to square with that inconvenient conglomeration of external facts that we call their environment.
Fig. 25. Interpetiolar Stipules.
That is why the goose-grass and the other stellate weeds have foliage of this minute character, instead of broad blades like the two seed leaves. But all plants of tangly growth do not attain their end in precisely the same manner. Sometimes one plan succeeds best and sometimes another. In most cases the originally round and simple leaf gets split up by gradual steps into several smaller leaflets. In the stellate tribe, however, the same object is provided for in a widely different fashion. Instead of the primitive leaf dividing into numerous leaflets, a number of organs which were not origi
nally leaves grow into exact structural and functional resemblance to those which were. Strictly speaking, in this whorl of six little lance-shaped blades, precisely similar to one another, only two opposite ones are true leaves; the other four are in fact, to use a very technical term, interpetiolar stipules. A stipule, you know, of course, is a little fringe or tag which often appears at the point where the leaf stalk joins the stem, and its chief use seems to be to prevent ants and other destructive insects from creeping up the petiole. But in all the stellate plants the two little stipules on each side of each leaf have grown gradually out into active green foliar organs, to supplement and assist the leaves, until at last they have become as long and broad as the original leaflets, and have formed with them a perfect whorl of six or eight precisely similar blades. How do we know that? you ask. In this simple way, my dear sir. The other Rubiaceæ — that is to say, the remainder of the great family to which the stellate tribe belongs — have no whorls, but only two opposite leaves; and we have many reasons for supposing that they represent the simpler and more primitive type, from which the stellate plants are specialised and highly developed descendants. But between the opposite leaves grow a pair of small stipules, occupying just the same place as the whorled leaflets in the goose-grass; and in some intermediate species these stipules have begun to grow out into expanded green blades, thus preserving for us an early stage on the road towards the development of the true stellates. Accordingly, we are justified in believing that in the whorls of goose-grass the same process has been carried a step further, till leaves and stipules have at last become almost indistinguishable.
There is, however, one way in which we can still distinguish the original true leaves of each whorl from the leaf-like stipules. Only two leaves out of the six ever have buds or branches proceeding from their axils; and this last token infallibly marks out for us which are the real primitive opposite pair, and which the spurious imitation.
What may be the use of the square stem it would be more difficult to decide. Perhaps it may serve to protect the plant from being trodden down and broken; perhaps by its angularity and stringiness it may render it unpalatable to herbivorous animals. This much at least is certain, that very few cows or donkeys will eat goose-grass. There is another large family of plants — the dead-nettle tribe — all of which have also square stems; and they are similarly rejected as fodder by cattle. Indeed, the very fact that the stellate tribe have become thus quadrangular, while the other and earlier members of the madder kind, like coffee and gardenia, have round stems, in itself suggests the idea that there must be some sufficient reason for the change, or else it would never have taken place; but, as in many other cases, what that reason may be I really cannot with any confidence inform you from my simple professional chair on the gate here. If I were only at Kew Gardens, now — well, that might be a different matter.
And now let us come down to the individual peculiarities of the goose-grass, and ask what is the use of the wee recurved prickles which you can see thickly scattered on the stalk and whorls by the aid of my pocket lens. You observe that they occur all along each angle of the stem, and around the edge and midribs of the leaflets as well. If you try to pull a bit of goose-grass out of the thicket entire, you will soon see the function they subserve. The plant, you notice, resists your effort at once; the little prickles catch securely on to the bushes and defeat all endeavours to tear it away. It is these prickles, indeed, which are the raison d’être of the goose-grass as a separate species: they mark it off at once from almost all the other members of the same group. There are many allied kinds of galium in England (for galium is the botanical name of the genus), with very similar leaves and flowers, but they all grow in shorter bunches and frequent less thickly populated situations. Goose-grass, however, has survived and become a distinct kind just in virtue of these very hooks. By their aid it is enabled to scramble for many feet over hedges and bushes, though it is but an annual plant; and it thus makes use of the firm stem of yonder hawthorn and this privet bush by our sides to raise its leaves into open sunny situations which it could never reach with its own slender stalk alone. Such an obvious improvement gives it an undoubted advantage in the struggle for life, and so in its own special positions it has fairly beaten all the other galiums out of the field. One of its common English names — Robin Run-the-hedge — sufficiently expresses the exact place in nature which it has thus adapted itself to fill and to adorn.
But how did the goose-grass first develop these little prickles? That is the question. Granting that their possession would give it an extra chance in the struggle for existence, if once they were to occur, how are we to account for their first beginning? In this way, as it seems to me. Viewed structurally, the stout little hooks which arm the stem and leaves are only thickened hairs. Now hairs, or long pointed projections from the epidermis, constantly occur in almost all plants, and in this very family they are found on the edges of the leaflets and on the angles of the stem among several allied species. But such hairs may easily happen to grow a little thicker or harder, by mere individual or constitutional variation; and in a plant with habits like the goose-grass every increase in thickness and hardness would prove beneficial, by helping the festoons to creep over the bushes among which they live. Thus generation after generation those incipient goose-grasses which best succeeded in climbing would set most seed and produce most young, while the less successful would languish in the shade and never become the proud ancestors of future plantlets. Even the less highly developed species, such as the wall galium and the swamp galium, have little asperities on the edge of the stem; but, as they need to climb far less than the hedgerow goose-grass, their roughnesses hardly deserve to be described as prickles. Our own special subject, on the other hand, being a confirmed creeper, finds the prickles of immense use to it, and so has developed them to a very marked extent. The corn galium, too, which clings to the growing haulms or stubble of wheat, has learnt to produce very similar stout hooks; while the wild madder, which I suspect is far more closely related to goose-grass than many other plants artificially placed in the same genus, has prickles of like character, but much larger, by whose aid it trails over bushes and hedges for immense distances.
After the leaves and stem we have to consider the nature of the flower. Look at one of the blossoms on the piece I gave you, and you will easily understand the main points of its structure. You notice that it consists of a single united corolla, having four lobes joined at the base instead of distinct and separate petals, while the centre of course is occupied by the usual little yellow knobs representing the stamens and pistil. Each goose-grass plant produces many hundreds of such flowers, springing in small loose bunches from the axils of the leaves. What we have to consider now is the origin and meaning of the parts which make them up.
Fig. 26. — Single flower of Cleavers.
We have already seen in dealing with the daisy that the really important organs of the blossom are the little central yellow knobs, which do all the active work of fertilising the ovary and producing the seeds. The stamens, as we then observed, manufacture the pollen, and when the pistil is impregnated with a grain of this golden dust the fruit begins to swell and ripen. But the corolla or coloured frill around the central organs, which alone is what we call a flower in ordinary parlance, shows that the goose-grass is one of those plants which owe their fertilisation to the friendly aid of insects. Blossoms of this sort usually seek to attract the obsequious bee or the thirsty butterfly by a drop of honey in their nectaries, supplemented by the advertising allurements of a sweet perfume and a set of coloured petals. So much knowledge about the functions of flowers in general we have already acquired; the question for our present consideration is this: What gives the goose-grass flower in particular its peculiar shape, colour, and arrangement?
First of all, you will notice that it has a united corolla — a single fringe of bloom instead of several distinct flower leaves. This marks its position as a very proud one in the
floral hierarchy; for you will remember that only the most advanced blossoms have their originally separate petals welded into a solid continuous piece. Once upon a time, indeed, the early ancestors of our little creeper had five distinct petals, like those of a dog-rose or a buttercup; but that was many, many generations since. In time these petals began to coalesce slightly at the base, so as to form a short tube as in the primrose; and, since this arrangement made it easier for the insect to fertilise the flowers, because he was more certain to brush his head in hunting for honey against the pollen-bearing stamens and the sensitive summit of the pistil, all the flowers which exhibited such a tendency gained a decided advantage over their competitors, and lived and flourished accordingly, while their less fortunate compeers went to the wall. So in the course of ages such tubular flowers, like harebells and heaths, became very common, and to a great extent usurped all the best and most profitable situations in nature. Among them were the immediate ancestors of the goose-grass, which had then regular long tubular blossoms, instead of having a mere flat, disk-shaped corolla like the one you see in the goose-grass before you. But, for a reason which I will presently tell you, in the goose-grass tribe itself the tube has gradually become shorter and shorter again, till at last there is nothing left of it at all, and the corolla consists simply of four spreading lobes slightly joined together by a little rim or margin at the base.
How do we know, you ask, that the goose-grass is descended from such ancestral flowers having a long hollow tube? Why may it not be an early form of tubular blossom, a plant which is just acquiring such a type of flower, rather than one which has once possessed it and afterwards lost it? Well, my dear sir, your objection is natural; but we know it for this reason. I told you some time since that the other great branch of the madder family, which had stipules instead of whorled leaves, was thereby shown to be a more primitive form of the common type than the stellate tribe, in which these stipules have developed into full-grown leaves. Now, all these tropical madder-like plants have large tubular blossoms, perfectly developed; so that we may reasonably infer the ancestors of the goose-grass had the same sort of flowers when they were at the same or some analogous stage of development. Moreover, amongst the stellate plants themselves there are several which still retain the long tubes to the blossom; and these are rather the less developed than the more developed members of the little group. Such are the pretty blue field-madder, which has a funnel-shaped corolla, and the sweet woodruff, which has bell-shaped flowers. But the galiums, which are the most advanced (or degraded) species of all, have the tube very short or hardly perceptible, and the more so in proportion as they are most widely divergent from the primitive type.
