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tween the majority of the ferns and the majority ot sper- 

 maphytes is adaptive, not a primitive character on the 

 part of the ferns is amply proved by the exceptions. 

 Some species in every tribe of Polypodiacese represented 

 at San Ramon are without chlorophyll in the epidermis. 

 The spicular cells of the Vittariese have already been 

 mentioned, and in four tribes, Davallieae, Asplenieae, 

 Polypodieae and Acrosticheae, are species which have car- 

 ried protective specialization deeper than the epidermis, 

 having a specialized hypodermis. In the majority of 

 ferns it is more correct to describe the epidermis as spe- 

 cialized in other directions than for protection than to 

 call it undifferentiated. In very numerous ferns it is, 

 indeed, a highly specialized photosynthetic tissue which 

 is not infrequently more specialized than any part of the 

 parenchyma. In Adiantum diaphanum an especially large 

 share of the photosynthesis falls to the epidermis, the 

 upper and nether epidermis being in direct contact in a 

 considerable part of the frond. It will be noticed that 

 the ones which have been mentioned in this connection 

 are nearly all terrestrial species, but some very large ter- 

 restrial species are like many epiphytes in the more or 

 less complete suppression of chlorophyll in the epidermis, 

 this being the case in Nephrodium ferox, N. cyatheoides 

 and the huge variety of Aspidium luzeanum. 



It is the mesophyll rather than the epidermis which 

 shows less specialization in the ferns than in the seed 

 plants. In the parenchyma the differentiation is espe- 

 cially backward. A complete and typically developed 

 palisade parenchyma does not occur, but layers which 

 are like it in compactness and more or less approaching 

 it in form and arrangement of the cells are found in many 

 species. According to its necessity, a hyaline epidermis 

 has been differentiated in one or more genera of every 

 tribe. This is usually found only beneath the upper epi- 

 dermis, but underlies the nether as well in Davallia solida, 

 Asplenium muscefolium, Polypodium incurviim and 



