— no 



others. This tissue is found only in xerophytes, and the 

 notion has some time had vogue that its function is to 

 act as a water-store. That this is not in general the 

 case I have pointed out elsewhere. Among the ferns, 

 the walls of the hypodermis are almost invariably so 

 thick that any change in size or form, which is neces- 

 sary if they are to give up any water, is quite impossible. 



On the other hand, there are a very few species pro- 

 vided with an evidently available store of water. Thus 

 Polypodium caudiforme, with two layers of non-collapsi- 

 ble cells under the upper epidermis, has one layer of col- 

 lapsible ones next the nether one. The walls of the green 

 parenchyma of Loxogramma iridifolia, Antrophyum reti- 

 culatum and Polypodium accedens are somewhat collapsi- 

 ble with loss of water, but not greatly so. In this direc- 

 tion again, it is Niphobolus of all our ferns in which 

 specialization has gone farthest. Niphobolus is likewise 

 the only genus having trichome-hydathodes. These hairs 

 are different in form, those of each of our species being 

 characteristic, our most xerophytic species, N. adnascens, 

 being glabrescent ; but they are all alike in insertion, each 

 hair growing in a pit which is practically filled by the 

 basal cell of the trichome. The basal cell is alive with 

 considerable evident contents. When the leaf is damp, 

 the contents fill the cell ; it can then absorb water from 

 the cells borne on it whether they are dead or alive, and 

 give water to the cells within. Judging by the high 

 turgor in Niphobolus leaves this movement must be fairly 

 active. When the outside of the leaf becomes dry, the 

 outer cells of these trichomes lose their water and 

 promptly draw on the basal cell. If the connection were 

 maintained, the basal cell would then supply itself from 

 the interior of the leaf. But this does not happen, be- 

 cause its protoplasm instead of keeping in connection 

 with the cells within and without shrink away from its 

 wall and contracts into a lump touching but one end of 

 the cell. A dead air space, or approximate vacuum, in 



