252 



A TEXTBOOK OP BOTANY 



[Ch. V, 8 



and the cap, though in some the cap is imitated by a poeket- 

 like cover, and the hairs l^y slender side roots. The reduc- 

 tion leads even to complete disappearance of roots in some 

 immersed Pondweeds, which absorb through their leaf and 

 stem surface, precisely as did their far-distant ancestors, 

 the Alga;. Herein we have a clear case of the widespread 



tendency for parts 

 rendered useless by a 

 change of habit to 

 disappear gradually, 

 often after lingering 

 long in a rudimentary 

 condition. 



There is in some 

 plants another and 

 very different correla- 

 tion between root 

 structure and water 

 hal:)it. Some com- 

 mon herbs when 

 grown in wet places 

 develop at the contact of air and water a loose open tissue 

 involving large intercellular spaces (aerenchyma), which 

 seem to transmit air to the under-water parts. In a half- 

 floating water plant called Jussirea repens (Fig. 176), some 

 of the roots develop this tissue immensely, becoming enlarged 

 to conspicuous white spindles which rise vertically to the 

 surface, whence they evidently take air for use of the under- 

 water parts. Tluis we have roots \\ith a new substitute 

 function, that of ac'ration. The Bald Cypress, a prominent 

 tr(>e of the southern swamps, develops from its roots remark- 

 al)le projections, or " knees," which commonly reach the water 

 surface, and are so constructed as to suggest their use as aerat- 

 ing organs for the roots, though such function has been tlenied. 

 Other structures of analogous sort are found in Mangroves 

 and elsewhere, as described in works upon water plants. 



Fig. 176. — Jussicea repenx : X |. The swol- 

 len structures are roots, composed chiefly of 

 aerenchyma ; when young their tip.s reach the 

 surface and they become filled with air, which 

 later they supply to the parts under water, 

 (.\fter Goebel.) 



