ROOTS AND RHIZOIDS 



519 



and salts through the latter; indeed, the rise of colored liquids has 

 been witnessed in the rhizoids of Polytrichum. When the rhizoids of 

 Catharinea are severed in the soil, the leaves wither precisely as they do 

 in seed plants when the roots are cut. Water drops appearing on cut 

 stem surfaces of Mnium 

 have been supposed to 

 indicate conduction and 

 hence rhizoid absorp- 

 tion. In Polytrichum 

 and in Catharinea the 

 rhizoids are intertwined 

 like the strands of a 

 rope, so that doubtless 

 water can ascend be- 

 tween them by capillar- 

 ity as well as within 

 them. In mat and 

 cushion mosses it is prob- 

 able that leaf absorp- 

 tion, facilitated by the 

 ready capillarity made 

 possible by close con- 

 tact between adjoining 

 shoots, is much more im- 

 portant than is rhizoid 

 absorption (p. 611). 



Rhizoids of algae and 

 of lichens. — Algae. — 

 Many small algae are 

 entirely unattached, and 

 move actively {Volvox) 

 or drift passively (Pleu- 

 rococcus). Other forms 

 are attached by the muci- 

 lage they exude. Some filamentous algae (as Ulothrix and Oedogonium) 

 are anchored by the basal cell, which may differ from the other cells in 

 shape and color ; such forms often are epiphytic on other water plants. 

 Rootlike rhizoids occur in Vaucheria, Botrydium, and Chara. Large 

 marine algae are attached to rocks by much-branched rhizoids (or 



Fig. 751. ^ a marine alga {Nereocystis), showing a 

 rhizoid complex or system of haptera, which serves as 

 a holdfast organ, fi.ting the plant firmly to a rocky sub- 

 stratum below sea level; note the stalk or stipe, whose 

 bladder-like expansion floats the leaflike organs at the 

 water surface. — From Coulter (Part I). 



