— Ill — 



the basal cell then protects the interior of the leaf from 

 evaporation. The protoplasm of the basal cell collapses 

 instead of maintaining its turgidity, because it loses 

 water outward faster than it can get it from within. 

 This condition must be due both to the very high turgor 

 of the mesophyll and to the unequal permeability of the 

 end-walls of the basal cell, their outer ends being pitted. 



Very many ferns have their vein tips hyaline and, as 

 a rule, the clear spots are hydathodes, clear because of the 

 absence of air-containing spaces. Such hydathodes are 

 found among other ferns in Meniscium, Arthropteris, 

 Nephrolepis, many Aspleniums, Hymenolepis, numerous 

 Polyp odiums and various others. White incrustations of 

 lime are regularly found on these hydathodes on some 

 species of N ephrolepis, and occasionally on various other 

 ferns. There are other hyaline vein-tips, as in Asplenium 

 subnormale, which are not active hydathodes, though per- 

 haps potential ones. 



Except as it is modified by correlation with other struc- 

 tural peculiarities, such as fineness of dissection of the 

 frond, the venation in general is decidedly closer in 

 species of arid than those of humid habitats. Anasto- 

 mosis of the veins makes the venation closer in effect, 

 and, as a general proposition, with many exceptions, 

 ferns with anastomosing veins are more xerophytic in 

 habitat than those with free veins. The frequent correla- 

 tion between large size and ampleness of frond and 

 reticulate venation is too obvious to need elaboration. 



The articulation of the stipe to the rhizome and of 

 the pinnae or segments to the stipe facilitate the reduc- 

 tion or removal of the leaf surface whenever necessary. 

 It is thus an adaptation to life where plants must some- 

 times endure a more or less prolonged want of water. 

 Like other adaptive characters, but in greater measure 

 than many, because it involves a deeper specialization, 

 it has a taxonomic value, as species, genera and even 

 larger groups have developed a constant adaptation to 



