. A textbook of botany for colleges and universities ... Botany. Reactions to external conditions. — Rhizoids are progeotropic,' prohy- drotropic, and apophototropic, thus agreeing with roots and differing greatly from root hairs. In Marchantia gemmae or fern prothallia, rhizoids may be induced at will on either side of the thallus by exposure to the proper stimuli (moisture, darkness, contact, etc.); after the rhizoids appear, their growth direction may be altered by altering their relation to light, moisture, or gravity.' When Lunularia is grown in solutions defi- cient in nitrogen or in pho
. A textbook of botany for colleges and universities ... Botany. Reactions to external conditions. — Rhizoids are progeotropic,' prohy- drotropic, and apophototropic, thus agreeing with roots and differing greatly from root hairs. In Marchantia gemmae or fern prothallia, rhizoids may be induced at will on either side of the thallus by exposure to the proper stimuli (moisture, darkness, contact, etc.); after the rhizoids appear, their growth direction may be altered by altering their relation to light, moisture, or gravity.' When Lunularia is grown in solutions defi- cient in nitrogen or in phosphorus, the rhizoids become greatly elongated, while the thallus is poorly developed, recalling the strong roots and the de- pauperate stems of xero- phytes. No rhizoids develop on Lunularia in pure water, but enough salts are dissolved from ordinary laboratory vessels to induce rhizoids abun- dantly. Very plastic liverworts are Riccia natans and R. luiescens, plants that grow either in soil or in water. Ordinary rhizoids develop in the soil (fig. 742), but in the water their place is taken by large and con- spicuous ventral scales (figs. 743-746). Rhizoids, Hke root hairs, are better developed in xerophytic than in hydrophytic forms, and aquatic species (as Riccia fluitans) commonly are without them. Rdle. — Liverwort and fern rhizoids obviously are anchorage organs, and generally they are believed to be organs of absorption also, although the proof therefor is not so conclusive as in the case of root hairs. Since the thallus is close to the ground and permeable, rhizoid absorption may be of minor value. The arguments for the absorptive role are (i) cementation to soil particles (fig. 1075) and modification in rhizoid ' From recent work it appears that the hydrotropism of rhizoids is much more pro- nounced than is their geotropism. ' 746 745 Figs. 745, 746. — A ven- tral scale from the water form of Riccia: 745, the tip of the scale, showing scattered mucilage cells {g)
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