Riiiznms 



25 



which grow horizontally around the support and so serve as anchor- 

 ing roots. Such roots probably do not perform the function of 

 absorption at all. 



In corn and a nunil)er of other nionocotyledonous plants roots 

 are produced at some of the lower aerial nodes of the stem. These 

 grow obliquely downward as unbranched air roots until they reach 

 the soil. After entering the soil they branch freely and serve an 

 important function as prop roots. In the banyan tree roots grow 

 vertically downward and into the soil from horizontal branches. 

 These roots become stiff and form a 

 system of supports which make it pos- 

 sible for a single tree to continue to 

 spread until it covers an enormous 

 area. 



Many tropical orchids which grow 

 upon the branches of other plants 

 where the conditions for absorption are 

 difficult have specially modified air 

 roots (Fig. 7). These roots are usually 

 shining white. Structurally they differ 

 from ordinary roots by having a layer 

 several cells thick outside of the cortex. 

 This is called the velamen and it ab- 

 sorbs water with great rapidity by 

 capillarity whenever dew or rain comes 

 in contact with it. Between the vela- 

 men and the cortex there is a single 

 layer of cells, the exodermis, the cell 

 walls of which, in most cases, are more 

 or less thickened. Some cells in this 

 layer, however, called transfusion cells, 

 have thin walls, and probably serve as 

 passageways for water from the velamen into the cortex. 



15. Rhizoids. —Roots are strictly organs of sporoph3i;es. Gameto- 

 phytes never have true roots but are supplied instead, in many cases, 

 with rhizoids. Rhizoids are filamentous structures only one cell 

 thick. In fern prothalli and in liverworts they are usually unicellular 

 and unbranched while in mosses they are branched and multicellular 

 but still only one cell thick. Many of the large marine algae and 

 some lichens are attached to the substratum by rhizoids. All 

 rhizoids serve as anchorage organs and many are probably also of 

 value in absorption. 



Fig. 7. — Sector of a cross- 

 section of an air root of an 

 orchid. F, velamen; E^x, exo- 

 dermis; C, cortex; £'n, endo- 

 dermis; VC, vascular tissue; 

 P, pith. 



