5 10 



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 \Cerato fhyttum^ U triculotj&r- 

 xfljid 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 , 

 ioo(~~(a.& in 

 Wolffia); in| 

 all three there 

 is a thallus 

 rather than a 

 leafy stem, 



FIG. 728. The tip 

 of a water root of a 

 water hyacinth (Eich- 

 horniaspeciosa), show- 

 ing the root pocket, 

 which fits over the root 

 like a glove finger; 

 considerably magni- 

 fied. 



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 role 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 fpund in various attached aquatics, such as 

 and Myriophyllnm. Horizontal branches give rise to hairless unbranchcd root 



FIG. 729. A tropical epiphytic orchid 

 (Epidendrunt ramosum), 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). 



I 



