510 



ECOLOGY 



root to a plant. Thus the modifications induced when soil roots are 

 grown in water (viz. reduction in size and in hair production) are seen 

 to be the usual features in aquatic roots. The 

 cause of reduction and of hairlessness in water 

 roots is unknown, though it is likely that the 

 conclusions reached in the case of soil roots are 

 applicable here. 



In some water plants, as Ceratophyllum, Utricularia, 



and Salvinia (fig. 897), roots are wanting and absorption 



is confined to 



the leaves and 



stems. The 



duckweeds 



may have sev- 

 eral roots (as 



in Spirodela), 



one root (as in 



Lemna), or no 



root (as in 



Wolffia); in 



all three there 



is a thallus 



rather than a 



leafy stem, 



and in the 

 rootless Wolffia, absorption takes place 

 in the same manner as in the algae. 



In water roots the outer layers do 

 not become impermeable with age, so 

 that absorption takes place through 

 the entire surface instead of through 

 the tips, only, as in soil roots. Water 

 roots are of no value as anchorage 

 organs, but they may assist in the 

 maintenance of equilibrium. Many 

 aquatic roots contain chlorophyll, and 

 it may be that food manufacture is 

 an accessory r61e of some importance. 

 An interesting feature of water roots 

 is the root pocket, a structure that fits 

 over the root tip like a glove finger 

 (fig. 728). Although root pockets are much more conspicuous than are the root 

 caps of soil roots, their advantage to the plant is less evident. Roots intermediate 

 between soil and water roots are found in various attached aquatics, such as Elodea 

 and Myriophyllum. Horizontal branches give rise to hairless unbranched roots 



Fig. 728. — The tip 

 of ii water root of a 

 water hyacinth (Eich- 

 hornia spsciosa), show- 

 ing the root pocket, 

 which tits over the root 

 like a glove finger; 

 considerably magni- 

 fied. 



Fig. 729. — A tropical epiphytic orchid 

 {Epidendrum ramosuni), showing aerial ab- 

 sorptive roots arising adventitiously from the 

 nodes, and mostly just above the leaves, 

 which exhibit distichous phyllotaxy (i.e. with 

 leaves alternating in two vertical rows, p. 549). 



