EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 351 



condensed, by marginal fusion of lobes and veins, that it appears 

 collectively as a unit, and so it seems to justify its designation col- 

 lectively as the " lamina " (Figs. 262-3). 



Explanatory terms for the three regions, into which the leaf appears to be 

 divided in many Dicotyledons, help in upholding this view of it as a branch- 

 system. The rachis springs laterally from the stem : and three parts of it may 

 be commonly distinguished by their form and function, as : 



(1) The hypo-rachis or leaf -base, to which the stipules are attached, if present ; 

 though frequently there are none. 



(2) The meso-rachis or petiole, which results from an extension of a middle 

 region intercalated between the leaf-base and the blade. If the intercalation 

 of the petiole begins above the extreme base the position of the stipules 

 suggests that a first pair of pinnae had been left behind in the process. 



(3) The epi-rachis, which embodies the whole of the distal branch-system. 

 It is composed of lateral rows of potential pinnae, or of these either separate 

 or fused in various degrees laterally, to form the flanges of a coherent blade or 

 lamina (Figs. 260-3). 



The value of such terms is that they fix the attention on the branch-system 

 as a whole, though its branching may be obscured, or even absent in many 

 flowering plants, especially where the leaves are small, 

 and in the Monocotyledons (Fig. 264). A diverse 

 development of the individual leaves of many Di- 

 cotyledons may accentuate the character of these three 

 regions, even in the leaves of the same individual 

 plant. For instance those of Ribes (Fig. 263) : for 

 they are differentiated in form according to their 

 function. To the left in the figure is the normal foliage 

 leaf, with its sheathing base, its petiole, and its 

 lamina, all present. In the leaf to the right the 

 protective hypo-rachis is largely developed, and the 

 other parts are vestigial : while the two middle figures 

 show the petiole, or meso-rachis, in various degrees 

 of interpolation between the other parts. Thus each 

 region of the leaf may be specialised to perform its 

 several functions : (1) bud protection, (2) adjustment 

 of leaf-mosaic to avoid overshadowing, and (3) ex- 

 posure of receptive surface for sunlight. But all 

 three involve parts of one rachis. 



The leaf of a typical Monocotyledon, such as Funkia (Fig. 264), presents a 

 distinction of sheathing base and blade similar to that in many Dicotyledons. 

 But the broad expanse of the latter originates not by fusion of pinnae, but by 

 lateral expansion of mesophyll between the parallel veins of the distal part of the 

 rachis, which is here unbranched. Thus we see that the blade of Funkia is not 

 the developmental homologue of that of an ordinary Dicotyledon, but rather 

 its analogue. 



Doubts may thus arise as to the strict correspondence between leaves of 

 Dicotyledons and Monocotyledons. But when we come to the leaves of Mosses 

 and Liverworts any doubts of strict homology with those of Flowering Plants 



Fig. 264. 



Funkia grandiflora, upper 

 part of petiole and ' ' iamnia ". 

 (Reduced after Mrs. Arber.) 



