HYDROPHYTE ASSOCIATIONS. 



175 



extremely enduring, and therefore diatoms are often found 

 in great deposits in the rocks, in some cases forming the 

 whole mass of rock. Associated with the diatoms are 

 numerous other plant and animal forms. 



132. Pond associations. — The word pond is used to indi- 

 cate stagnant or slow-moving waters. In such waters free- 

 swimming plants of all groups are associated. Of course 

 the alga? are well represented, but even the highest plants 

 are repre- 

 sented by the 

 duckweeds, 

 which are very 

 commonl y 

 seen in the 

 form of small 

 green disks 

 floating on the 

 surface of the 

 water, which 

 they frequent- 

 ly cover with 

 great masses 



(see Fig. 156). It should be observed that the floating and 

 submerged positions result in a difference in light-relations. 

 The floating forms may be regarded as light forms, being 

 exposed to the greatest amount of light. The submerged 

 forms are shade plants, and the shading becomes greater 

 as the depth of the water is greater. It must not be sup- 

 posed that submerged plants can live at any depth, for 

 soon a limit is reached, beyond which the light is not 

 intense enough to enable plants to work. 



It has been noticed that this complete water habit has 

 affected plants in many ways. For instance, the duck- 

 weeds are related to land plants with root, stem, and leaves, 

 but they have lost the distinction between stem and leaf, 

 and the body is merely a fiat leaf-like disk floating upon 



Fig. 136. A section through the body of a duckweed (Lemna), 

 showing the air spaces (a) which make it buoyant, the 

 origin (r) of the simple dangling root, and the pockets 

 and l) from which new plants bud out, and in which 

 flowers are developed. 



