520 



ECOLOGY 



haptera) of considerable size, 

 entiated in structure (fig. 751). 



though they are simple and undififer- 

 Anchorage is thought to be the chief 

 role of Such rhizoids, and it is an im- 

 portant role, since marine algae are 

 especially prevalent along rocky coasts, 

 where wave action is violent. The 

 hairlike rhizoids of Bolrydium (fig. 732) 

 and of Chara may be of value also 

 in absorption, as seems to have been 

 proven in the case of Chara. Rhizoid 

 formation in algae is determined largely 

 by contact, especially with rough sur- 

 faces. 



Lichens. — Lichens usually are at- 

 tached to the substratum by rhizoids 

 (or rhizines) which may have hairlike 

 or discoid tips. These rhizoids are of 

 great importance as holdfast organs, 

 since lichens usually grow in exposed 

 situations, where they might otherwise 

 be blown away. Lichens are able to 

 grow directly on bare rocks, and this 

 is partly because the rhizoids exude 

 substances which corrode the sub- 

 stratum; especially is this true of 

 calcareous lichens, some of which 

 become almost entirely embedded within the rock. Possibly lichens 

 are able also to absorb water and solutes through their rhizoids. 



Concluding remarks on roots and rhizoids. — Often the progress of 

 evolution has been along the line of a division of labor. Such seems to 

 have been the case in the evolution of organs of absorption and of anchor- 

 age. Among the algae the entire plant body takes part in the absorp- 

 tion of water, salts, and gases, though primitive anchorage organs are 

 represented by the haptera. In the bryophytes, rhizoids are generally 

 developed, and probably they are organs of absorption as well as of 

 anchorage, though the plant body still takes an important part in ab- 

 sorption. In the ferns and seed plants, the roots are the chief anchorage 

 organs, and the young roots with their hairs are the chief organs of water 

 absorption, while the leaves are largely without absorptive efficiency. 



Fig. 752. — A plant of Botrydium^ 

 showing the much-branched colorless 

 subterranean rhizoid system and the 

 globular green aerial portion ; the plant 

 is a coenocyte, i.e. multinucleate but 

 without internal cell walls; consider- 

 ably magnified. — From Rosiapinski 

 and WOEONIN. 



