by Grant Allen
Fig. 27.
Flower of Field-madder.
Why, however, should a flower which was once tubular have lost its tube? If it was an advantage to acquire such a long narrow throat, must it not also be an advantage always to retain it? That depends entirely upon the nature of the circumstances to which the plant must adapt itself. Now the fact is, the original madder group seems to have had large and showy flowers, which were fertilised by regular honey-sucking insects, such as bees and butterflies and humming-bird hawk-moths. These are tropical shrubs, often of considerable size, and of very different habits from our little goose-grass. But in the temperate regions, since the earth has begun to cool into zones, some of these rubiaceous plants have found out that they could get along better by becoming little creeping weeds; and these are the stellates, including our present friend. Accordingly they have mostly given up the attempt to attract big honey-sucking insects whose long proboscis can probe the recesses of jasmine or woodbine, and have laid themselves out to please the small flies and miscellaneous little beetles, which serve almost equally well to carry their pollen from head to head. Now the flowers which specially cater for such minor insects are usually quite flat, so that every kind alike can get at the honey or the pollen; and that, I fancy, is why the goose-grass and so many of its allies have lost their tubes. They are, in fact, somewhat degenerate forms, descended from highly adapted tropical types, but now readjusted to a humbler though more successful grade of existence.
Closely connected with this question is the other and very interesting problem of their colour. Why is goose-grass white? For the very same reason — because it wishes to attract all sorts of little insects impartially. For this purpose white is the best colour. Almost all flowers which thus depend for fertilisation upon many different species of winged visitors are white. And, indeed, the sort of colour in each kind of stellate flower (as in all others) depends largely upon the sort of insects it wishes to attract. Thus the little field-madder, which has a long tube and is fertilised by honeysuckers of a high type, is blue or pink, as all the family once was, no doubt, before it began to bid for more vulgar aid. Then the lesser woodruff, or squinancy-wort, whose tube is shorter, has white cups tinged with lilac. The goose-grass and most of its neighbours, whose flowers have undergone still greater degeneration, are simply white, because they wish to please all parties equally, and white is of course the most neutral colour they could possibly assume. Finally, the lady’s bedstraw, which has no tube, depends upon little colour-loving beetles for fertilisation, and, like many other beetle flowers, it is bright yellow.
This order of degradation exactly reverses the upward order of chromatic progress; for, as flowers advance in type, they pass from yellow, which is the lowest colour, through white, pink, red, and lilac, to purple and blue, which are the highest. And when through any special cause they begin to retrogress, they pass backward through the same stages in inverse order.
Again, you may have observed that I said just now the primitive ancestor of the goose-grass had five petals. But the present united corolla has only four lobes instead of five, and it is this arrangement, apparently, which has gained for the whole tribe the name of stellate. Now the tropical Rubiaceæ, which we saw reason to believe represent an earlier stage of development than the goose-grass group, have usually five lobes to the corolla; and in this respect they agree in the lump with the whole great class of dicotyledonous plants to which they belong. Therefore we may fairly conclude that to have four lobes instead of five is a mark of further specialisation in the stellates; in other words, it is they that have lost a lobe, not the other madder-worts that have added one. This, then, gives us a further test of relative development — or perhaps we ought rather to say of relative degeneration — among the stellate tribe. Wild madder, whose flowers are comparatively large, has usually five lobes. Yellow crosswort has most of its blossoms four-lobed, interspersed with a few five-lobed specimens. Goose-grass occasionally produces large five-lobed flowers, but has normally only four lobes. The still smaller skulking species have almost invariably four only. In fact, the suppression of one original petal seems to be due to the general dwarfing of the flower in most of the stellate tribe. The corolla has got too small to find room for five lobes, so it cuts the number down to four instead. This is a common result of extreme dwarfing. For example, the tiny central florets of the daisy ought properly to be pinked out into five points, representing the five primitive petals, but they often have the number reduced to four. So, too, in the little moschatel, the outer flowers of each bunch have five lobes, but the central one, which is crowded around and closely jammed by the others, has regularly lost one in every case.
Fig. 28. — Strawberry and Asperula.
To show Inferior and Superior Ovaries.
There is just one more peculiarity of the goose-grass blossom which I must not wholly overlook. You see this rough little bulb or ball beneath the corolla, covered with incipient prickles? That is the part which will finally grow into the fruit, after some friendly insect has brought pollen on his legs from some neighbouring flower to impregnate the ovary of this. Now, what I want you to notice is the fact that the future fruit here lies below the corolla — below the flower, as most of us would say in ordinary language. But if you think of a strawberry, a raspberry, or a poppy, you will recollect that the part which is to become the fruit there grows above the corolla, and that the petals are inserted at its base. This last is the original and normal position of the parts. How and why, then, has the ovary in the goose-grass kind managed to get below the petals? Well, the process has been something like this: When the flowers were tubular they were surrounded by a tubular calyx, and the ovary stood in the middle of both. But in the course of time, in order to increase the chances of successful fertilisation, the calyx tube, the corolla tube, and the ovary in the centre all coalesced into one solid piece — grew together, in fact, just as the five petals had already done. So now this little bulb really represents the calyx and ovary combined; while the corolla, only beginning to show at the top, where it expands into its four lobes, looks as if it started from the head of the fruit, whereas in reality it once started at the bottom, but has now so completely united with the calyx in its lower part as to be quite indistinguishable. Thus the fruit is not in this plant a mere ripe form of the ovary, but is a compound organ consisting of the calyx outside, and the ovary inside, with the tube of the corolla quite crushed out of existence between them.
Fig. 29.
Fruit of Cleavers.
Last of all, let us look at the prickly fruit itself in its ripe condition. Some small fly has now fertilised the head with pollen from a brother blossom; the corolla and the stamens have fallen off; the embryo seeds within have begun to swell; the mother plant has stocked them with a little store of horny albumen to feed the tiny plantlets when they are first cast forth to shift for themselves in an unsympathetic world; and now the fruit here is almost ready to be detached from the stalk and borne to the spot where it must make its small experiment in getting on in life on its own account. Before I tell you how it manages to get itself transported free of cost to a suitable situation, I should like you to observe its shape and arrangement. It consists of two cells or carpels united in the middle, and each of these contains a single seed. Once upon a time there were several cells, as there still are in some of the tropical Rubiaceæ, and each cell contained several seeds, as is the case with many of the southern species to the present day. But when the stellate tribe took to being small and weedy, they gave up their additional seeds and limited themselves to one only in each cell. This is another common result of the dwarfing process, and it is found again in all the daisy tribe and in the umbellates, such as fool’s parsley. To make up, however, for the loss in number of the seeds in each fruit, the number of fruits on each plant is still enormous. How many there are on a single weed of goose-grass I have never had the patience to count, but certainly not less than several hundred. You might find it
a nice amusement for a statistical mind to fill up this lacuna in our botanical knowledge.
Most of the stellate plants have simple little fruits without any special means of dispersion, but in the goose-grass the same sort of prickles as those of the stem and leaves are further utilised for carrying the seed to its proper place. You know seeds have many devices for ensuring their dispersion to a distance from the mother plant. Some are surrounded by edible pulp, as in the case of the raspberry or the gooseberry; and these are swallowed by birds or animals, through whose body they pass undigested, and thus get deposited under circumstances peculiarly favourable to their germination and growth. Others have little wings or filaments, as in the case of the dandelion or the valerian; and these get blown by the wind to their final resting-place. Yet others, again, are provided with hooks or prickles, like the burr and the hounds-tongue, by whose means they cling to the wool of sheep, the feathers and legs of birds, or the hair of animals, and thus get carried from hedge to hedge and rubbed off against the bushes, so as to fall on to the ground beneath. Now this last plan is especially well adapted for a plant like the goose-grass, which lives by straggling over low brambles and hawthorns, for it ensures the deposition of the seed in the exact place where the full-grown weed will find such support and friendly assistance as it peculiarly requires. Accordingly, we may be sure that if any half-developed goose-grass ever showed any tendency to prickliness on its fruit, it would gain a great advantage over its neighbours in the struggle for existence, and the tendency would soon harden under the influence of natural selection into a fixed habit of the species. Is there any way in which such a tendency could be set up?
Yes, easily enough, as it seems to me. You remember the outer coat of the fruit is really the calyx, and this calyx would be naturally more or less hairy, like the original leaves. We have only to suppose that the calyx hairs followed suit with the stem hairs, and began to develop into stiff prickles, in order to understand how the burr-like mechanism was first set up. Supposing it once begun, in ever so slight a degree, every little burr which succeeded in sticking to a sheep’s legs or a small bird’s breast would be pretty sure, sooner or later, of reaching a place where its seeds could live and thrive. It is from this habit of cleaving or sticking to one’s legs that the plant has obtained one of its English names — cleavers. Moreover, to make the development of the burr all the more comprehensible, many of the other galiums have rather rough or granulated fruits, while one kind — the wall galium — which in England has smooth or warty fruit, has its surface covered in southern Europe with stiff hairs or bristles. Another English galium besides goose-grass has hooked bristles on its fruit, though they are not so hard or adhesive as in our own proper subject. Thus the very steps in the evolution of the bristly fruit are clearly preserved for us to the present day in one or other of the allied species.
On the other hand, the very similar little corn galium, which has prickles on its stem and leaves to enable it to cling to the growing straw in the wheat-fields, has no hooks at all upon its fruit. Instead of a burr it produces only little rough-looking knobs or capsules. At first sight, this difference between the plants is rather puzzling, but when we come to consider the peculiar habits of the corn galium we can see at once the reason for the change. Like most other cornfield weeds, it blossoms with the wheat, and its seed ripens with the mellowing of the shocks. Both are cut down together, and the seed of the galium is thrashed out at the same time as the grain. Thus it gets sown with the seed corn from year to year, and it would only lose by having a prickly fruit, which would get carried away to places less adapted for its special habits than the arable fields. It has accommodated itself to its own peculiar corner in nature, just as the goose-grass has accommodated itself to the hedgerows and thickets. So again, in the wild madder, the fruit, instead of becoming rough and clinging, has grown soft and pulpy, so as to form a small blackish berry, much appreciated by birds, who thus help unconsciously to disperse its seeds. Each plant simply goes in the way that circumstances lead it, and that is why we get such infinite variety of detail and special adaptation even within the narrow limits of a single small group.
And now I think you are tired both of your seat on the gate and of my long sermon. Yet the points to which I have called your attention are really only a very few out of all the facts which go to make up the strange, eventful life-history of this little creeper. If you had but leisure and patience to hear me I might go on to point out many other curious details of organisation which help us to reconstruct the family pedigree of the goose-grass. There is not a single organ in the plant which does not imply whole volumes of unwritten ancestral annals; and to set them all forth in full would require not a single hour but a whole course of ten or twenty sermons. Still, I hope I have done enough to suggest to you the immense wealth of thought which the goose-grass is capable of calling up in the mind of the evolutionary botanist; and I trust, when you next get your clothes covered with those horrid little cleavers, you will be disposed to think more tenderly and respectfully than formerly of an ancient and highly developed English weed.
V. THE ORIGIN OF WHEAT.
Wheat ranks by descent as a degenerate and degraded lily. Such in brief is the text which this paper sets out to prove, and which the whole course of evolutionary botany tends every day more and more fully to confirm. By thus from the very outset placing clearly before our eyes the goal of our argument, we shall be able the better to understand as we go whither each item of the cumulative evidence is really tending. We must endeavour to start with the simplest forms of the great group of plants to which the cereals and the other grasses belong, and we must try to see by what steps this primitive type gave birth, first to the brilliantly coloured lilies, next to the degraded rushes and sedges, and then to the still more degenerate grasses, from one or other of whose richer grains man has finally developed his wheat, his rice, his millet, and his barley. We shall thus trace throughout the whole pedigree of wheat from the time when its ancestors first diverged from the common stock of the lilies and the water-plantains, to the time when savage man found it growing wild among the untilled plains of prehistoric Asia, and took it under his special protection in the little garden plots around his wattled hut, whence it has gradually altered under his constant selection into the golden grain that now covers half the lowland tilth of Europe and America. There is no page in botanical history more full of genuine romance than this; and there is no page in which the evidence is clearer or more convincing for those who will take the easy trouble to read it aright.
Fig. 30.
Head of Wheat.
Moreover, the case of wheat is a very interesting one, after the case of the daisy and of cleavers, because it exhibits a different order of evolution, that namely of continuous degradation. While the daisy has gone constantly up, and while the goose-grass has fallen but a little after a long course of upward development, the grasses generally have from the very first exhibited a constant and unbroken structural decline.
The fixed point from which we start on our inquiry is the primitive and undifferentiated ancestral flowering plant. Into the previous history of the line from which the cereals are ultimately descended, I do not propose here to enter. It must suffice for our present purpose to say dogmatically that the flowering plants as a whole derive their origin from a still earlier flowerless stock, akin in many points to the ferns and the club-mosses, but differing from them in the relatively important part borne in its economy by the mechanism for cross-fertilisation. The earliest flowering plant of the great monocotyledonous division (the only one with which we shall here have anything to do) started apparently by possessing a very simple and inconspicuous blossom, with a central row of three ovaries, surrounded by two or more rows of three stamens each, without any coloured petals or other ornamental adjuncts of any sort. I need hardly here explain even to the unbotanical reader that the ovaries contain the embryo seeds, and that they only swell into fertile fruits after they have been d
uly impregnated by pollen from the stamens, preferably those of another plant, or at least of another blossom on the same stem. Seeds fertilised by pollen from their own flower, as Mr. Darwin has shown, produce relatively weak and sickly seedlings; seeds fertilised by pollen from a sister plant of the same species produce relatively strong and hearty seedlings. The two cases are exactly analogous to the effects of breeding in and in or of an infusion of fresh blood among races of men and animals. Hence it naturally happens that those plants whose organisation in any way favours the ready transference of pollen from one flower to another gain an advantage in the struggle for existence, and so tend on the average to thrive and to survive; while those plants whose organisation renders such transference difficult or impossible stand at a constant disadvantage in the race for life, and are liable to fall behind in the contest, or at least to survive only in the most unfavourable and least occupied parts of the vegetal economy. Familiar as this principle has now become to all scientific biologists, it is yet so absolutely necessary for the comprehension of the present question, whose key-note it forms, that I shall make no apology for thus once more stating it at the outset as the general law which must guide us through all the intricacies of the development of wheat.
