ROOTS AND RHIZOIDS 



517 



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) ; 746, a few 

 of the scale cells, showing 

 chloroplasts; both figures 

 highly magnified. 



Fig. 747. — A moss 

 plant (gametophyte), 

 showing the aerial stem 

 (s) with leaves (/) in sev- 

 eral vertical rows; the 

 underground organs are 

 branched structures, the 

 rhizoids (r). 



