ECOLOGY 



In Washington and in British Columbia the hemlock often germinates on 

 stumps, logs, or standing trees, and has a subsequent history somewhat compa- 

 rable to that of Ficus in the tropics. 



The banyan habit is illustrated on a small scale by Selaginella, especially in moist 

 chamber cultures, where there develop rhizophores with numerous root hairs 



(figs. 735, 896). In the mangrove, 

 roots are put forth much as in 

 Ficus, but they branch profusely 

 in the air and spread out laterally 

 (fig. 741). So abundant are these 

 roots in mangrove swamps that 

 they form a dense network over 

 the soil, and are of much impor- 

 tance in supporting the numerous 

 branches, owing to the early death 

 of the basal part of the primary 

 trunk. The factors operative in 

 the production of prop roots are 

 quite obscure; in Selaginella, water 

 seems to be an important stimulus. 



Liverwort and :ern rhizoids. 



Structure. The rhizoids 

 of liverworts and of fern pro- 

 thallia commonly are color- 

 less, unicellular outgrowths 

 of special external cells, and 

 may or may not eventually be 

 cut off therefrom by a cell 

 wall (fig. 742). They closely 

 resemble root hairs in struc- 

 ture, but usually they are of 

 much greater length and their 

 protoplasm is more thinly 

 disseminated. In the Mar- 

 chantiaceae, rhizoids are of 

 two kinds, plane rhizoids as 

 above described and peg rhi- 

 zoids, in which the cell wall 

 grows out internally into peg- 

 like or antler-like projections. 



744 



FIGS. 742, 743, 744. Plants of a liverwort, 

 Riccia: 742, an individual grown with soil con- 

 tact and showing a luxuriant but thin and much- 

 lobed thallus and an abundant growth of rhizoids 

 ( r )> 743. a portion of the individual figured in 

 742, transferred to water, and allowed to develop 

 there for some weeks; 744, the plant figured in 

 743 at a later stage ; note that in 743 and 744 the 

 thallus is much smaller and thicker than in 742, 

 and that flat ventral scales (v) have taken the 

 place of rhizoids (r) ; through the death of the old 

 part of the thallus (o) the new lobes become de- 

 tached at the sinuses, resulting in the vegetative 

 origin of a number of inividuals from one; all 

 figures somewhat magnified. 



The plane rhizoids occur chiefly on the younger apical and midrib 

 portions, while the peg rhizoids occur on the older marginal portions. 



