Of all known plants none are more weird and singular than the Cacti, with their angular succulent stems, armed with spines, and the absence of leaves. In many instances the flowers are large, showy, and beautiful. These plants are numerous in the hotter and drier parts of America. "Sometimes globular, sometimes articulated, sometimes rising in tall polygonal columns, not unlike organ-pipes." Now and then attaining a very gigantic size, occasionally so small that "they get between the toes of dogs." In the similarly dry and arid tracts of Africa these plants are absent, but their place is occupied by species of Euphorbia, which resemble in form and habit the Cacti of America. In our woodcut (fig. 66) is represented one of these Euphorbias, growing amongst rocks at the Cape of Good Hope, which is entirely dissimilar from our common wood spurge and the other Euphorbias of temperate regions, whilst it is scarcely to be distinguished from a cactus, except that they have usually small and inconspicuous flowers. The two families of plants are widely separated from each other, almost as far as possible for plants to be, and yet the resemblance is so great that in the absence of flowers it is difficult to believe that they do not belong to one family. Not only do they resemble each other, but they are also "imitated" by plants of another family, the Asclepiadaceae, of which the species of Stapelia might equally be attributed to Cacti or Euphorbia. These plants may be seen growing together in the "succulent house" at Kew.
<!-- image -->Fig. 66.—Euphorbia, resembling a cactus growing amongst rocks in Damara Land.
Before leaving these succulent plants we may also instance certain of the lily family, small aloes of the genus Haworthia, in which the fleshy leaves grow close to the ground in the form of a rosette. In this instance the resemblance approaches to that of the house-leek family (Crassulaceae), further removed than even Euphorbia and Cactus, for one belongs to Monocotyledonous and the other to Dicotyledonous plants.
If from these general features we turn to individual plants, we shall find the number of examples greatly increased. Any one who has had an extended experience will appreciate the difficulties which constantly arise in determining even the "order" of an unknown plant in the absence of flowers or fruit. Turn, for instance, again to the Euphorbiaceae, and compare one of the species of Phyllanthus, with flattened phyllodes, as Phyllanthus falcata with a similar structure in a species of the Buckwheat family (Polygonaceae). Here, in an unusual form, a striking mimetic resemblance will be encountered.
Or, if we have only the young condition, without flowers or fruit, of such a floating plant as Jussiaea repens, one of the Onagraceae, we shall at once be struck with its resemblance to a similar condition of an Euphorbiaceous plant (Phyllanthus fluitans), and, at the same time, with such a cryptogam as Salvinia rotundifolia. In our figures of these three plants the resemblance is less striking than in the plants themselves (fig. 67). All of them float on the water, under similar conditions, in different parts of the world.
Dr. Berthold Seemann speaks of having seen, in the Sandwich Islands, a variety of Solanum (S. nelsoni) which looked for all the world like a well-known Buettneraceous plant of New Holland (Thomasia solanacea), "the resemblance between the two widely-separated plants being quite as striking as that pointed out in Bates's 'Naturalist on the Amazons,' between a certain moth and a humming bird."
The circumstance has also been alluded to many times that one of our highest botanical authorities of his day figured a species of Veronica, without flowers or fruit, as a conifer. This serves to remind us how many plants there are, scattered through various families, in which the form and arrangement of the foliage is so much like that of a cypress, that it is almost impossible to distinguish them from the foliage alone. Others again are nearly identical with some of the larger species of Lycopodiums, cryptogams allied to ferns and mosses. We have figured one example of a compact composite plant (Azorella selago) for comparison with one of the club-mosses (Lycopodium compactum). Other instances might have been selected from remote families, in which the same resemblance is sustained, so that from the figures it would scarce have been possible to determine whether they were "club-mosses" or not.
<!-- image -->fig. 67.--Young plants of (a) Salvinia, (b) Jussiaea repens, (c) Phyllanthus.
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Foliage is hardly so satisfactory for comparison, except in cases where the leaves have a strongly marked character, nevertheless we may suggest two or three of the most striking. The leaves of the planes and the maples have a coincidence of type. The pinnate leaves of some of the Oxalidaceae closely resemble, even in their sensitive nature, some of the mimosa family. Or, if we instance individual species, the leaves of the common holly are imitated in some of the evergreen oaks. Some of these latter, especially varieties of the common Quercus ilex, are very like the olive. The trifoliate leaves of the wood-sorrel are very similar to those of the white clover, and both are represented, except that the leaves are four-lobed instead of three, in the cryptogamic genus Marsilea. There is an antarctic species of Caltha, or marsh marigold, in which the leaves strongly remind us of the Venus's fly-trap, and this originated its specific name (Caltha dionaefolia.) The digitate leaves of some of the cultivated species of Aralia might easily be mistaken for those of the castor-oil plant, although the former is allied to the ivy, and the latter to the far-distant Euphorbiaceae. Another striking likeness in cultivated plants may be found in two variegated leaved species, with bright crimson veins. These are an Acanthaceous plant (Gymnostachyum Verschaffeltii), and one of the Apocynaceae (Echites rubrovenosa). The size, form, colour, and mode of venation is almost identical. Numerous examples of pairs of plants resembling each other, chiefly in foliage, have been exhibited at the meetings of the Linnean and other scientific societies.
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The inflorescence sometimes has a puzzling resemblance in one plant, or series of plants, to others with which they have no natural affinity. Some of the large African species of Polygala might easily be mistaken for Papilionaceae: So again the Fig-marigolds (Mesembryanthemum) have a general likeness to the flowers of composite plants. A more extraordinary instance is in a genus of umbelliferous plants, of which two Australian species (Actinotus) are strangely like ox-eye daisies (fig. 71). Our pretty little yellow "rock rose" (Helianthemum) reminds one of the yellow species of Potentilla, or the wood crowfoot (Ranunculus), and yet all three flowers belong to widely-separated natural orders. An instance also occurs to us in which an experienced botanist misnamed the flowers of Coffea bengalensis, as those of Tabernaemontana, although they belong to families with no family connection. So also the inflorescence of Dodecatheon, nearly allied to the Cyclamen of the gardens, is not uncommonly mistaken for that of the dog's tooth violet (Erythronium dens-canis), which it imitates in size, form, colour, and even in the bending backwards of the petals. Many of the myrtle family are excellent imitators of the Rosaceae, and the rotate flowers of the fragrant white jasmins of India remind one strongly of Apocynaceae.
<!-- image -->Fig. 71.—Actinotus.
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The most striking instances of recurrence of type will be found amongst fruits, and perhaps the most numerous. Before "mimicry" was thought of in animals or plants, it had been remarked, as a singular coincidence, that the seeds of an Indian tree, Mesua ferrea, were like chestnuts. These seeds are not only alike in size, form, and colour, but also in character, so that they are eaten as a dessert fruit, in a similar manner. The Indian tree belongs to the same family as the gamboge and mangosteen, whilst the chestnut finds a place with the oak, in a family far removed. Somewhat alike to these, but less striking, is the seed of the horse chestnut, the fancied resemblance being perpetuated in the name.
<!-- image -->Seeds of Mesua ferrea, natural size.
There is also great similarity between some of what are termed indehiscent legumes of the Leguminosae, and drupes of the Rosaceae; as, for instance, the pod of the tonquin bean and the fruit of an almond. The families are too closely allied, however, to give much weight to these resemblances. Perhaps the pod-like fruits of some of the caper family (Capparidaceae) and their similarity to those of some of the Leguminosae is more noteworthy. The form, size, and colour of some small gourds, of the cucumber family, such as the colocynth and the orange gourd, approximate to the fruit of the orange.
The winged fruits of the maples, with the seed at one extremity and a veined wing at the other, is a type of "samara" which is found repeated again in other families. It occurs in a genus of Polygalaceae, which is found chiefly in tropical South America. Our figure is Securidaca tomentosa (fig. 74). The same form is found again in Malpighiaceae, of which the species are mostly South American. This is represented by Heteropterys laurifolia (fig. 74); and yet again in the Phytolaccaceae, the same kind of samara is found in at least two genera, of which we have illustrated Gallesia goranema and Seguiera floribunda. These four illustrations are from three natural orders, all separate from each other and from the maples, and yet, not only is the size and form the same, but also the veining in the wings. So deceptive is the resemblance between these fruits, that only dissection and analysis could determine one from the other.
<!-- image -->Fig. 74.—Samara of Securidaca tomentosa, Heteropterys laurifolia, Gallesia goranema, Seguiera floribunda.
Another type of samara is that of the elm, in which the seed occupies the centre, surrounded by a wing. Our common forms are those of the common elm (Ulmus campestris) and the wych elm (Ulmus montana), the latter being the largest. This form of fruit is imitated in Ptelea trifoliata, a tree of the Rutaceae, and in a species of Hiraea, one of the Malpighiaceae. In like manner there is more than a merely superficial resemblance, but almost identity, although the families to which the trees belong have no close relationship (fig. 75).
<!-- image -->Fig. 75.—Samara of (a) Ulmus campestris, (b) Ulmus montana, (c) Ptelea trifoliata, (d) Hiraea.
Winged seeds, as distinguished from the winged fruits just alluded to, are highly developed, and of considerable size in the family of trumpet flowers (Bignoniaceae). One of the best-known forms (Calosanthes indica) is given in the woodcut (fig. 76). The membrane which surrounds the seed is beautifully delicate and transparent, and is a favourite object with microscopists. This type of seed is represented again in the cucumber family, in which winged seeds are rare, and is, in fact, almost an imitation of the seed of the Calosanthes. In our figure (fig. 77) it has been reduced by about one-third, so as to bring it within limits of the page. It does not differ more from the seed of one of the Bignoniaceae than these seeds differ amongst themselves. In another family (Apocynaceae), similar winged seeds occur (as in Aspidosperma excelsum from Guatemala), although it is not a special feature in that family for the seeds to be expanded in a membranaceous wing.
Every schoolboy is acquainted with the downy crest of the achenes, or fruits, of the dandelion and thistle. This crest of delicate hairs, or pappus, is common in composite plants, but it is not confined to them. From the annexed woodcut (fig. 78) it will be seen that one of the forms, with the crest sessile, is reproduced in three other families, viz., in Sarcostemma (a) one of the Asclepiadaceae, in Echites scabra, one of the Apocynaceae (b), in the willow herb or Epilobium, one of the Onagraceae (c), and in the milk thistle, Silybum marianum, one of the Compositae. There is more difference in the character of the seeds themselves than in the crest which surmounts them.