128 



KNOWLEDGE 



[Junk, 1902. 



The Australian Casuarinas are dicotyledons with in- 

 complete flowers ncaa-ly related to the oak, hazel and 

 other Cupiilifcnc, but in outward appearance they have 

 ji singular rescnibhuicc to the horse-tails, a family of 

 Ci-yptoganis. One of the Gyinnospcrnis or cone-bearing 

 class. Ephedra, also presents the same jointed appearance 

 so chai-actcristic of EquisotacesD. Growing in marshy 

 2)laces very like those affected by Ecjuisctum we find 

 the mare's-tail Hippurus, a lloweriug plant allied to the 

 fuchsia family, but externally resembling Equisctum in 

 its joiuted stem and whorlcd leaves. A familiar instance 

 of the same kind of homomoi-phism is Equisetum 

 sylvaticum, which might almost be describeid as a 

 Lilliputian fir-tree. The little flowers of the water 

 ranunculus look exactly like miniature water lilies, while 

 the leavas and flowei-s of Caltha palustris simulate the 

 3'ellow Nuphar so much that in some parts of the counti-y 

 tho niai'sh marigold is known as the water lily. The 

 specific name of another aquatic, Lymnanthemum 

 nympheeoides, indicates a peculiarity of the same kind. 

 Leaf analogies are frequent among aquatic plants ; the 

 orbicular, peltate leaf of the Indian cress occurs, for 

 example, in Hydrocotyle, Nelumbium, and others. The 

 brown colour and translucence of Potamogeton, Myrio- 

 phyllum, and other aquatics assimilates them to the 

 fronds of Laminaria and other sea/-weeds. 



A grass-like habit is jissumed by some plants. This 

 character is attained in the meadow vetchling by the 

 arrested development of the compound leaves and the 

 great elongation of the stipules. Lathyrus nissolid has 

 the stipules minute, but the phyllodes or leaf-like petioles 

 impart the grass-like character. A moss-like habit occurs 

 in a gi-eat many plants belonging to very different 

 families ; thus the wiry stem of the purging fiax reminds 

 one of the seta of Polytrichum. The pearlwort of the 

 walls, many alpine saxifrages, pinks and gentians 

 present very much the appearance of mosses, e.g., Silene 

 acaulis, Saxifraija hrijoides, S. hyp/ioides, Arenaria 

 Cherleri, etc. The sub-species Sasifraga geum is another 

 instance of leaf analogy. The generic name Pyrola 

 implies a fancied resemblance of the leaves to those of 

 the pear tree. Certain leaf-types frequently recm-, the 

 rough broadly tongue-shaped leaf of the bugloss, for 

 example ; hence the very common specific appellation 

 echioides. The nettle-leaved bell-flower reproduces the 

 foliage of Urtica and the sinuate leaf of the oak appears 

 m several families. 



Parasitic phanerogams like RaiBesia commonly exhibit 

 the fungoid character in a marked degree. In their 

 internal structure, colouring, spore-like seeds and other 

 characters they approximate closely to the fungi. 



As examples of homomorphism between closely allied 

 plants may be mentioned the false oaf, which so sti'ik- 

 ingly resembles the cultivated species, and the barren 

 strawberry which agrees so closely with the cultivated 

 strawberry of our gardens. 



Although it is only under exceptional circumstances 

 that a flower is likely to mimic another blossom closely, 

 vague general resemblances are not uncommon, such as 

 that between the rock-rose and the buttercup, between 

 the milkwort and the vetch, and between Vei'onna and 

 Valerianflla. A more decided likeness is that of the 

 garden annual Collinsia to the butterfly blossoms of the 

 pea tribe. This case is peculiarly instructive since the 

 homomorphism can be traced to its cause. The butterfly- 

 like corolla of Leguniinosae seems to have afforded the 

 pattern after which a number of flowers have been 

 fashioned. The Papilioneicese are adapted to bees rather 

 than to butterflies or moths, and the pollen is applied 



to the ventral surface of the insect, the essential organs 

 being lodged in the carina or pouch formed by the two 

 lower petals. Among the Scrophulariaceas to which 

 Collinsia belongs, the pollen is commonly sj)riukled on 

 the back of the insect and the stamens are contained in 

 tho upper lip of the corolla; Collinsia is, however, 

 exceptional ; the stamens are lodged within the lower lip 

 of the flower and the pollen is applied to the ventral 

 surface of the bee. Here the resemblance is evidently 

 an indirect result brought about by the flowers of 

 Collinsia having become adapted to the same class of 

 visitors as the Papilionaceje, via., bees which have their 

 brushes or baskets of hair for collecting pollen attached 

 to tho abdomen. Where two flowers are very like insects 

 are apt to mistake the one species for the other, but 

 this will not involve any loss if there is an interval 

 between their periods of blossoming. 



Homomorphic likenesses are not confined to homo- 

 logous organs; an organ of one plant sometimes exhibits 

 a very perfect resemblance to a different organ on some 



Fio. 11. — The Duti'liman's Pipe CAristolovhia siphoj. 



other plant. Thus Aiistolochin sijiJw. the Dutchman's 

 pipe, so-called from the appearance of its flowers, has a 

 perianth singularly like the leaf-pitchers of Nepenthes, 

 and the curious little nectaries of Nigella might almost 

 be compared with the pitchers of the Australian 

 insectivorous plant Cephalotus. As the Aristolochias 

 imprison small dipterous insects in their flowers these 

 instances favour to some extent Henslow's idea that 



Fig. 12. — Slower of Aristolochia Fio. 13.— Pollen-masses 



trilolata and Leaf-pitcher of of Orchid and Asclepiad. 



Nepenthes. 



both flowers and pitchers have arisen by hypertrophy 

 caused through the irritation set up by insects. 



The homomorphism of the Orchids and Asclepiads is 

 specially interesting because of the objection to the 

 Darwinian theory that it presents; the coincidence is 

 certainly unfavourable to the notion of fortuitous 

 variation. The orchids and asclepiads agi-ee in producing 

 pollinia or pollen-packets which attach themselves to the 

 bodies of insects and are thus transferred from flower 

 to flower. Although the two flowers differ gre*tly in 



