1 64 SUBMERGED LEAVES [ch. 



feeble, as in the case of submerged plants, the epidermal cells 

 are kept in a constant state of turgescence, and hence their 

 growth takes a uniform course resulting in regularity of form. 

 But, on the other hand, when transpiration is active, as in 

 land life, the current is subject to great variations which react 

 upon the form of the epidermal cells and produce sinuosity. It 

 is scarcely possible to submit such a theory to direct proof, but 

 it seems to the present writer that it is at least consistent with 

 the fact, established at a much later date than Mer's work, that 

 Monocotyledons with their rectangular epidermal cells, are in 

 general, though with many exceptions, 'sugar-leaved' and 

 weak transpirers, while Dicotyledons, with their epidermal cells 

 often resembling a Chinese puzzle, are 'starch-leaved' and 

 strong transpirers^. 



The epidermal cells of submerged leaves differ from those 

 of air leaves not only in form but also in contents. Chlorophyll 

 grains, which are generally described as absent from the epi- 

 dermis of terrestrial plants, are often present in great abundance 

 in this tissue in submerged leaves^. Treviranus^, nearly a cen- 

 tury ago, alluded to the lack of distinctively epidermal charac- 

 ters or, to use his own expression, the "absence of an epider- 

 mis " in the case of the lower surface of the leaf of Potamogeton 

 crispus^ while Brongniart^, a few years later, observed the pre- 

 sence of chlorophyll in the leaf epidermis of P. lucens. Subse- 

 quently, epidermal chlorophyll has been observed widely among 

 aquatic plants^, though there are certain exceptions, such as 

 Callitriche^, In some cases, e.g. Zostera^ Cymodocea^ Posidonia'^ 

 the epidermis is actually the part of the leaf richest in green 

 corpuscles. The presence of chloroplasts does not constitute, 

 however, so absolute a difference from land plants as is some- 

 times assumed, since it has been shown that chlorophyll grains 

 can be found in the epidermis of the green organs of the 



1 Stahl, E. (1900). 2 Schenck, H. (1886). 



^ Treviranus, L. C. (1821). ^ Brongniart, A. (1834). 



5 Chatin, A. (1855I), etc. ^ Schenck, H. (1886). 

 ' Sauvageau, C. (1890^). 



