. A textbook of botany for colleges and universities ... Botany. Si6 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 i
. A textbook of botany for colleges and universities ... Botany. Si6 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 femrhizoids. — 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 ofiE 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 744 Figs. 742, 743, 744. — Plants of a Uverwort, 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 C**); 743« ^ portion of the individual figured in 742, transferred to water, and allowed to develop there for some weeks; 744, the plant figured in 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 t
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