328 PLANT LIFE. 



food by means of the weak light in a situation where there is 

 no danger from lack of water. 3. Floating or partly sub- 

 mersed plants, either free or attached. Many of the filamen- 

 tous algae and diatoms float free at the surface. The chief 

 characteristics of the higher floating plants which root in the 

 mud are these : their floating leaves are simple, little branched 

 or not at all, roundish or elliptical in form, leathery, and the 

 surface not easily wetted ; stomata are present only on the 

 upper surface, and the leaf stalks are adapted in length 

 to the depth of the water in which they grow ; the woody 

 tissues are either entirely absent or poorly developed, be- 

 cause there is no occasion for the transportation of water, 

 nor need of rigidity, since the medium in which they grow 

 supports most of the weight. 



445. Light. Green water plants are limited in their 

 distribution by the depth to which light can penetrate water. 

 This does not exceed even in pure waters four or five hundred 

 meters. No seed plants have been found at a greater depth 

 than thirty meters, and few algae at a greater depth than 

 forty meters. Plants which are brought up by dredging 

 from lower depths than this are usually those which have 

 been detached and sunk. 



446. The temperature of the water is very much less sub- 

 ject to variation than that of the air, never falling, except 

 at the surface, below 0.5 C.* 



447. The movements of the water are of much importance 

 to plants in bringing air and food materials to them. These 

 movements are wave movements, or surf, and currents. 

 Plants growing within the limits of wave action are often 

 damaged or torn away by the waves. The Sargasso Sea is 

 marked by an accumulation of such plants, mainly of brown 



* The minimum temperature of the deeper water is usually stated as 4 

 C. , but many observations upon Lake Mendota by Birge have shown that 

 in winter it falls nearly to zero, even at a depth of eighteen meters. 



