126 GEORGE HENSLOW [februaky 



Gkass-Leaved Type. — Herbaceous plants growing thickly together, 

 and so preventing each other from spreading out their blades hori- 

 zontally, have often assumed a grass-like foliage. Hence a caespitose 

 habit has induced a vertical position in the foliage, so that the leaf has 

 acquired a narrow linear form, the anatomical details following suit by 

 its developing stomata, etc., on both sides. Lathyrus nissolia, which 

 grows among grass, has this type of leaf. Thrift, Pinks and Carna- 

 tions, Sedges and the Bog Asphodel, as well as Grasses, have similarly- 

 formed leaves. Those belonging to dicotyledons retain the reticulated 

 venation, the branches starting from near the base at an acute angle, 

 thus more or less imitating the parallel venation of the monocotyledons. 



Specialised Mimetic Organs. — Leafless but foliaceous stems, 

 closely mimicking true leaves, occur in widely-separated orders having 

 no affinity. Thus Xylophylla (Euphorbiaceae) may be compared with 

 Euscus (Liliaceae) ; while of leafless but winged stems the following are 

 good examples : — Bossioea scolopendria, Genista sagittalis, and species of 

 Acacia (Leguminosae). 



Climbing Plants. — Mimicry is well seen among these ; as, for 

 example, between the tendrils of a vine or passion-flower, which con- 

 sist of metamorphosed flowering branches, and those of a pea, which 

 are homologous with a leaf. In Dissochaeta, aerial roots undertake 

 the same function, as do the slender branches of species of Strychnos. 



Ascidiform Type. — Perhaps no better illustration of plant mimicry 

 could be given than that between the pitcher of Cephalotus follicularis 

 and that of species of Nepenthes. These two genera are monotypic, and 

 therefore imply a long and now lost ancestry. The general appearance 

 of the mimetic pitchers is precisely the same in both. There is a 

 similar " lid," a pitcher of the same shape, with an inrolled margin, 

 glands sunk into the surface of the lining of the pitcher, which exter- 

 nally carries a fringed guide from bottom to top. Yet, while the 

 pitcher of Cephalotus is a metamorphosed leaf-blade, that of Nepenthes 

 is developed out of a water-gland situated at the apex of the blade, this 

 latter taking no part in its formation whatever. 



Such complete mimicry as this is quite as astounding as any 

 between two insects, whose forms and colours are alike, or between the 

 kangaroo mouse and the genus Mus. 



Conclusion as to Mimetic Vegetative Organs. — The above- 

 mentioned cases are but samples of what may be called a general 

 principle in nature ; which is, that since the living protoplasm is of one 

 and the same kind in all beings, as far as we know, where a certain 

 feature is evolved, a similar one may be expected under similar condi- 

 tions, and a mimetic organ is the result ; that is, so far as the conditions 

 of the structure will allow. Thus as the kangaroo mouse mimics a true 

 mouse, but retains its pouch and insectivorous teeth ; so a linear 

 dicotyledonous leaf imitates a grass blade, but retains its branching- 

 venation. 



