x] LEAF ANATOMY 1 3 1 



collected in a sterile and often fragmentary condition, and it 

 has thus become a matter of importance to systematists to be 

 able to identify them even when no organs of fructification are 

 present. It might have been expected that the examination of 

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

 form and all live completely submerged in a fairly uniform 

 environment, would reveal a monotony of internal structure. 

 But this expectation is far from being realised. Duchartre 1 

 showed in 1872 that the genera Cymodocea (Fig. 84, p. 125) and 

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

 one another, even in the absence of the flowers and fruit, on 

 anatomical grounds alone. This conclusion was carried much 

 further by Sauvageau 2 , who proved, as a result of detailed and 

 critical studies of the anatomy of the marine Phanerogams, that 

 (except among the Halophilas) the anatomy of the leaf gives 

 sufficient data for their exact generic and even specific deter- 

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

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

 Sauvageau pointed out, for instance, that the development of 

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

 Thalassia and Halophila, and that it is thus impossible to 

 regard this mechanical system merely as an adaptive response to 

 the milieu. The differences that are displayed by the different 

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

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

 conceivable that each of the detailed distinctions between the 

 closely related types of anatomy met with in the leaves of these 

 marine Angiosperms, is to be interpreted as having some 

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

 some 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- 

 characters which seem definitely related to submerged life. In 



1 Duchartre, P. (1872). 



2 Sauvageau, C. (iSgo 1 ), (iSgo 2 ), (iSQO 3 ) and (iSgi 1 ). 



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