[] LEAF ANATOMY 131 



:ollected in a sterile and often fragmentary condition, and it 

 las thus become a matter of importance to systematists to be 

 Lble to identify them even when no organs of fructification are 

 )resent. It might have been expected that the examination of 

 he leaves of these plants, which show great similarity in external 

 brm and all live completely submerged in a fairly uniform 

 ;nvironment, would reveal a monotonv of internal structure. 

 But this expectation is far from being realised. Duchartre^ 

 ;howed in 1 872 that the genera Cymodocea (Fig. 84, p. 125) and 

 Zostera (Figs. 85 and 86, p. 128) could be distinguished from 

 )ne another, even in the absence of the flowers and fruit, on 

 matomical grounds alone. This conclusion was carried much 

 "urther by Sauvageau^, who proved, as a result of detailed and 

 ;ritical studies of the anatomy of the marine Phanerogams, that 

 except among the Halophilas) the anatomy of the leaf gives 

 ;ufficient data for their exact generic and even specific deter- 

 nination. The variation occurring in the leaf structure is illus- 

 rated in Figs. 84, p. 125, 85 and 86, p. 128, 88 and 89, p. 132. 

 ?auvageau pointed out, for instance, that the development of 

 he lignified fibres differs markedly in the three genera, Enhalus, 

 Thalassia and Halophila^ and that it is thus impossible to 

 egard this mechanical system merely as an adaptive response to 

 he milieu. The differences that are displayed by the different 

 ipecies afford, indeed, another example of the fixity and lack 

 )f utility so often observed in specific differences; for it is not 

 ronceivable that each of the detailed distinctions between the 

 :losely related types of anatomy met with in the leaves of these 

 narine Angiosperms, is to be interpreted as having some 

 lefinite 'survival value,' though it may be broadly true that 

 ;ome structural variations are more suited to life in a boisterous 

 sea and others to existence in calmer waters. 



But though we cannot explain the different types of skeletal 

 system of the leaves on adaptive grounds, there are other leaf- 

 rharacters which seem definitely related to submerged life. In 



^ Duchartre, P. (1872). 



2 Sauvageau, C. (1890I), (iSgo^), (iSQO^jand (1891I). 



92 



