by Grant Allen
Fig. 20. Fruit of Wild Strawberry.
Fig. 21. Fruit of White Potentilla.
One may sum up the common points of the strawberry and the barren strawberry somewhat as follows: Both have tall leaves of three leaflets, raised on an elevated leaf-stalk, whereas most of their other congeners have many leaflets. Both have white flowers, whereas most of their other congeners have them yellow. Both have short tufted stems; in both the leaves are clothed with silky down; and in both the leaflets are regularly toothed at the edge in the self-same manner. On the other hand, they differ from one another almost exclusively in the matter of their fruit. Now, as we shall proceed to see, it is comparatively easy to produce the change whereby a dry fruit becomes a succulent one, and it is also comparatively easy to produce any one single change unaccompanied by others; but it is comparatively difficult to produce the whole set of changes whereby the two strawberries differ alike from all their congeners. So, if we are going to make a new genus, Fragaria, with a Latin name at all, we ought to make it include both the true strawberry and the barren strawberry, while we relegate to the genus Potentilla all the other less closely related kinds. But perhaps we shall do better if we lump them all together in a single genus, considering that, after all, the barren strawberry does not differ more from the remainder of the potentillas than many of these differ from one another among themselves.
And now, how did the edible strawberry get developed from its barren ally? Well, if we take the fruit of any potentilla, we shall find that it consists of several small, dry, one-seeded nuts, so tiny that they look themselves like seeds, crowded on a thick receptacle or flower stalk, without any signs of redness or succulence. In some potentillas, however, as the fruit ripens, this receptacle becomes a little spongy, something like the hull of a raspberry, only without its pulpy character. It is a common tendency of fruits to develop such pulpiness, and sometimes they do so quite suddenly by apparently spontaneous variation, as when an almond tree has been known to produce peach-like fruits. But no fruit will permanently acquire such a succulent character unless it derives some benefit by doing so: the change, once set up, will only be perpetuated by natural selection if it proves of advantage to the plants which happen to display it. Has it done so in the case of the strawberry?
A strawberry, as we all know, consists of a swollen red receptacle or end of the flower-stalk, dotted over with little seed-like nuts, which answer to the tiny dry fruits of the potentilla. Suppose any ancestral potentilla ever to have shown any marked tendency towards fleshiness in the berry, what would happen? It would probably be eaten by small hedgerow birds, who would swallow and digest the pulp, but would not digest the seed-like nuts embedded in its midst. Hence the nuts would get carried about from place to place and dropped by the birds in hedgerows or woods, under circumstances admirably adapted for their proper germination. Supposing this to happen often, the juiciest berries would get most frequently eaten, and so would produce hearty young plants oftener than those among their neighbours which simply trusted to dropping off casually among the herbage. Again, the birds like sweetness as well as pulpiness, and those berries which grew most full of sugary juices would be most likely to attract their attention. Once more, the brightest-coloured fruits would be most easily seen among the tall foliage of the hedgerows, and so those berries which showed any tendency towards redness of flesh would be sure to gain a point in attractiveness over their greener rivals. Thus at last the strawberry has grown into the fruit that we know so well, by constant unconscious selection of the little hedgerow birds, exerted at once in favour of the pulpiest, the sweetest, and the ruddiest berries.
It is noticeable, too, that in all these particulars what happens to the strawberry happens also in a hundred other independent cases. Wherever animals are to be enticed by plants, sugar is sure to be developed to entice them. It is so developed in the honey of flowers, in the extra-floral nectaries used for attracting ants, and in the sweet secretion by which the pitcher plants allure flies into their murderous vessels. So, too, bright colour is commonly employed to advertise the sweet material, as in the petals of flowers, the skin of fruits, and the pink or purple patches on the lips of the pitcher plants. On the other hand, the particular way in which these allurements are displayed by the strawberry is very different from that adopted by almost all other fruits. In the closely allied raspberry, the pulpiness and colour are produced in the outer coat of the little nutlets themselves, and the receptacle assumes the form of the hull, which we pull out of the fruit and throw away. In the plum, there is only one such berry, inclosing a single seed. But in the strawberry, the separate fruits remain always hard and dry, and it is only the receptacle which holds them that swells out into the bright-coloured and juicy edible portion.
It very seldom happens, however, that a plant which has diverged from another in one point remains constant in all other points. In the strawberry this is almost the case, for it hardly differs at all in any particular, save its fruit, from its ancestor, the white potentilla; and that is good evidence, it seems to me, that the two plants cannot very long have separated from one another. Yet even here there are a few inconspicuous lateral differences. Most notable of these are the variations in the flower. Though to a casual observer the two blossoms look almost identical, and the plants can only readily be identified when in fruit, a botanical eye has never any difficulty in distinguishing the one from the other. The petals of the barren strawberry are usually short and narrow, the flowers scarcely open into more than a cup shape, and there is a good deal of yellowish or reddish colour about the receptacle and the base of the stamens. In the true wild strawberry, on the other hand, the petals are usually larger, rounder, and purer white, the flowers open into a wide saucer shape, and there is no yellow or red in the centre of the blossom. Perhaps one may best account for these changes by supposing that the true strawberry has still further progressed in insect fertilisation. This would sufficiently explain the purer white of the petals and the loss of such relics of the primitive yellow hue as still remained in the barren strawberry. But it is also probable, I think, that the barren strawberries represent the remnants of the old ancestral race which have not yet been lived down by the newer strawberry type, but which are gradually undergoing progressive degradation; hence their half-opened flowers — often self-fertilised — their smaller and degenerate petals, and their general unattractiveness of outward appearance. It is difficult to compare the blossom of a true wild strawberry with that of a barren strawberry without immediately catching the obvious suggestion that the one is going upward towards higher development and the other downward towards general degeneracy.
In some other respects the strawberry plant equally shows itself the nobler species of the two. Its leaves are usually larger and more erect, its stem taller and straighter, its root-stock less fluffy and not so creeping. Moreover, if it really descended from the white potentilla, or from some closely allied common ancestor, it has certainly far outstripped its progenitor in the race for the possession of the world, for the white potentilla, or barren strawberry, is apparently a strictly European species, found from Sweden and Ireland to the Crimea and the Caucasus, but the true strawberry is a much more cosmopolitan plant, being found in almost all the temperate regions of the world, from Siberia and Scotland to Vancouver’s Land, and from the Arctic Regions to the Andes of Chili. This is quite what one would expect under the circumstances; for while the seed-like fruits of the white potentilla could only fall on the ground close to the mother plant, and so could disperse themselves very slowly over a single continent, the little nuts of the strawberry could be carried by birds from land to land, across the severing ocean or the intervening tropical region. Thus the old degenerate type is now apparently dying out in northern and western Europe; but the progressive and advancing strawberry is making its way steadily, like a colonising race, round the entire girdle of the two temperate regions.
The strawberries are, as yet, it would seem, a relative
ly new race, and so they have not, so far, split up into any very marked or distinctive separate species. Still they have even now assumed several minor forms, worthy at least to be distinguished as nameable varieties. The most divergent of these, as might be expected, is the Chilian pine strawberry, for in the southern hemisphere the imported strawberry, carried over at first, no doubt, by some weather-driven bird, has found itself in the midst of a very different environment from that which surrounds it in the hedgerows and meadows of its European home: and to this environment it has had to adapt itself in several minor but obvious particulars. An almost equally distinct variety is the white Alpine strawberry, which has quite lost the native blushing ruddiness of the lowland fruit. Curiously different in another way is the hautboy, a taller plant, with fewer and larger blossoms and a richer flavour, chiefly distinguished by the separation of its sexes on distinct plants, for here the stamens grow on one vine, and the pistils, or embryo fruits, on another. In order to make the berries swell and ripen, it is necessary to plant both sorts together, and then the fertilising insects unconsciously carry the pollen from the staminate flowers to the sensitive surface of their pistillate neighbours, and so assist the efforts of the gardener in setting the fruit. In the great American market gardens it is usual to plant one row of ‘barren’ flowers for every three rows of ‘fertile’ ones, leaving the insects to do the rest. At present none of these varieties can be said to have developed into what old-fashioned botanists used to call ‘a good species,’ for fertile cross-breeds can still be readily produced between them all by artificially fertilising the pistils of one sort with pollen taken on a camel’s-hair brush from the stamens of another. The possibility of fertile hybridisation in such a manner shows that the plants have not long diverged from the common central stock. But after they have long been exposed to varying circumstances and acted upon by natural selection, they will probably become so different from one another in a variety of small particulars that the hybrids will no longer prove fertile, owing to the want of sufficient similarity between the respective ancestral lines. Perfect fertility is only possible between individuals which still retain all the principal traits of a common ancestral form. Curiously enough, one existing variety, the Himalayan strawberry, has actually reverted to the primitive yellow flowers of its cinquefoil allies.
On the other hand, if the strawberries ever really live down the white potentillas, so that the latter race dies out altogether, then the distance between the genus Fragaria and the genus Potentilla will be far greater than it is at the present day. We are lucky enough at this moment to be able to trace the close connection between one rather abnormal potentilla (the barren strawberry) and the true strawberry itself. But if the barren strawberry and the Himalayan kind were to disappear we should have no link between the yellow-flowered, five-leaved, dry-fruited cinquefoil and the white-flowered, three-leaved, succulent-fruited strawberry. In nature, as it now stands, the ‘missing link’ is fortunately not yet missing. Though still essentially a potentilla in all important points, it yet approaches so nearly to the true strawberry that only rather close observation enables us to perceive their differences in certain stages of their existence. What thus happens now with the strawberry has doubtless happened at one time or another with every new species of plant or animal; but the special interest of this case consists in the fact that here, in all probability, we still have the parent type living on in a degraded form side by side with its more advanced and developed descendant.
IV. CLEAVERS.
Fig. 22. — Goose-grass or Cleavers.
Sitting here on the gate that leads into the Fore Acre, I have just disentangled from my nether integuments a long trailing spray of clinging goose-grass, which has fastened itself to my legs by the innumerable little prickly hooks that line the angles of its four-cornered stem. It is well forward for the time of year, thanks to our wonderfully mild and genial winter; for it is already thickly covered with its tiny white star-shaped flowers, which have even set here and there into the final mature stage of small burr-like fruits. Goose-grass, or cleavers, as we ordinarily call it, is one of the very commonest among English weeds, and yet I dare say you never even heard its name till I told it to you just now; for it is an inconspicuous, petty sort of plant, which would never gain any attention at all if it were not for its rough clinging leaves, that catch one’s fingers slightly when drawn through them, and often obtrude themselves casually upon one’s notice by looping themselves in graceful festoons about one’s person. Now I am glad to have got you button-holed here upon the gate, because I can tell you all about the goose-grass as we sit on the top bar without risk of interruption; and I dare say you will be quite surprised to learn that a very interesting and historical plant it is after all, in spite of its uninviting external aspect. You will find that its prickly leaves its square stem, its white flowers, and its odd little fruit all tell us some curious incident in its past evolution, and are full of suggestiveness as to the general course of plant development. Here is our weed in abundance, growing all along the hedgerow by our side, and clambering for yards from its root over all the bushes and shrubs in the thicket. Pick a piece for yourself before I begin, and then you can follow my preaching at your leisure, with the text always open before you for reference and verification.
Fig 23.
Seedling of Cleavers.
Of course goose-grass had not always all its present marked peculiarities. Like every other living thing, it has acquired its existing shape by slow modification from a thousand widely different ancestral forms. One of the best ways to discover certain lost links in the pedigree of plants or animals is to watch the development of an individual specimen from the seed or the egg; for the individual, we have all often been told, to some extent recapitulates in itself the whole past history of its race. Thus the caterpillar shows us an early ancestral form of the butterfly, while it was still a wingless grub; and the tadpole shows us an early ancestral form of the frog, while it was still a limbless mud fish. So, too, the chick hatching within the shell goes through stages analogous to those of the fish, the amphibian, the reptile, and the bird successively. In just the same way young plants pass through a first simple shape which helps us to picture to ourselves what they once were — what, for example, the ancestors of the goose-grass looked like, long before they were goose-grasses at all. Now here in my hand I have got a young specimen in its very earliest stage, which closely reproduces the primitive type of its first progenitors, a million ages since. Goose-grass is an annual weed: it dies down utterly every autumn, and only reproduces itself by seed in the succeeding spring; but this year the weather has been so exceptionally warm and summer-like that thousands of young plants have sprouted from the seed ever since Christmas; and among them is the specimen which I have just picked, and which you may have for examination if you will take the trouble. Look into it, and you will see that its two first leaves are quite unlike the upper ones — a phenomenon which frequently occurs in seedling plants, and with which you are probably familiar in the case of the pea and of the garden bean. But this difference is always a difference in one direction only; the first leaves which come out of the seed are invariably simpler in shape and type than all the other leaves which come after them. In the language of science they are less specialised; they represent an earlier and undeveloped form of leaf — nature’s rough sketch, so to speak, while the later foliage represents the final improvements introduced with time, and perfected by the action of natural selection.
These large oval leaves which you see in the seedling are mere general models or central ideas of what a leaf should be; they are quite unadapted to any one special or definite situation. They are not divided into many little separate leaflets, or prolonged into points and angles, or gracefully vandyked round the edges, or beautifully cut out into lace-like patterns, or armed at every rib with stout defensive prickles, like many other leaves that you know familiarly. Their outline is quite simple and unbroken; they preserve f
or us still the extremely plain ancestral form from which such different leaves as those of the horse-chestnut, the oak, the clover, the milfoil, the parsley, and the holly are ultimately derived. An expanded oval, something like this, is the prime original, the central point from which every variety of foliage first set out, and from which they have all diverged in various directions, according as different circumstances favoured or checked their development in this, that, or the other particular. Just as a single little cartilaginous mud-haunter — a blind and skulking small creature, something like a lancelet, something like a tadpole, and something like the famous ascidian larva — has gradually evolved, through diverse lines, all the existing races of beasts, birds, reptiles, and fishes, so too a single little primæval plant, something like these two lowest leaves of the goose-grass, has gradually evolved all the oaks and elms and ashes; all the roses, and geraniums, and carnations; all the cabbages, and melons, and apples, which we see in the world around us at the present day. And, again, just as the larval form of the ascidian and of the frog still preserves for us a general idea of that earliest ancestral vertebrate, so too these larval leaves of the goose-grass, if I may venture so to describe them, still preserve for us a general idea of that earliest dicotyledonous plant.
