90 A. H. Graves, 



a. Epidermis. 



(1) Chloroplasts. 



An interesting point brought out by a study of the eiDidermis, 

 and shown especially well in the living condition, is the fact that 

 here most of the photosynthesis is carried on, for the cells contain 

 large numbers of chloroplasts (Text-fig. 12). The discussion of the 

 causes of this condition, being of an ecological nature, will be post- 

 poned for the present. 



(2) Marginal Teeth. 



The teeth, mentioned above, p. 88 (PL VI, fig. 26; Text-fig. 11), 



have already been observed in the leaves of 



Ruppia maritima bySauvageau(1891, II, p. 209), who 



says, "a son sommet [i. e. of the blade] la plupart 



des cellules terminales se prolongent en dents 



arrondies composees de 1—2-3 cellules. Des dents 



semblables, mais plus espacees, se retrouvent sur 



les bords lateraux du limbe et font legerement 



saillie." In a young stage, as in PL VII, fig. 39, the 



teeth appear mostl}^ at the end of the leaf primor- 



Portton" of" epi- dium; later they may be found down the margins 



dermis of leaf, of the leaf anywhere from | to | of the whole 



stowing cliloro- distance from the apex. Finally, in the adult leaf 



piasts. Drawn ^ -^ ' 



from living leaf. they rarely extend more than \ of the distance 



X 335. from the apex. 



(3) Secretion Cells. 



In the epidermis I have observed also the cells referred to by 

 Sauvageau (1891, II, p. 209) as " cellules secretrices," containing, in 

 alcoholic material, a granular or a homogeneous content of a 

 brownish color, and somewhat larger than the ordinary epidermal 

 cells (PL VI, fig. 30 ; PL III, fig. 8). They appear to be identical in 

 nature with those of the stem. Although Sauvageau asserts that they 

 are most abundant at the edges and at the apex of the leaf, yet 

 they appear to me often to increase in numbers towards its base, 

 including that part which adjoins the sheaths, as well as in the 

 sheaths themselves (PL VI, fig. 25). According to Sauvageau 

 these peculiar cells occur also in Posidonia, Cymodocea and Halo- 

 dule. He has ascertained the presence in them of tannin, probably 

 in combination. In my permanent slides their contents were almost 

 invariably granular, assuming, with the triple stain, either a crimson 

 or a brilliantly refracting yellow color — -more often the latter. 

 With ferric chloride the contents stained a brownish black and the 

 same result was given on treatment with potassium bichromate. 



