HYDROPHYTE ASSOCIATIONS. 181 



there is a definite relation to soil in which roots or root-like 

 structures are embedded. Associations of this type contain 

 the greatest variety of plants of all ranks. In these asso- 

 ciations are found algae, mosses, fern plants, pondweeds, 

 water lilies, etc. (see Figs. 158, 159, 160, 161). Pondweeds 

 and water lilies may be taken as convenient types of high 

 grade plants which grow in such conditions. 



In the first place, it will be noticed that they are in- 

 clined to social growths, great numbers of individuals 

 growing together and forming what are known as lily 

 ponds or pondweed beds, although in the small lakes of 

 the interior where pondweeds abound in masses, they are 

 more commonly known as " pickerel beds/' If the petiole 

 of a lily pad be traced down under the water, it will be 

 found to arise from an intricate mass of thick, knotted 

 stems. So extensively do these stems (rootstocks) in the 

 mucky bottom branch that they are able to give rise to 

 close set masses of leaves. 



Water lilies and pondweeds may also be compared to 

 show the effect of the floating habit in contrast with the 

 submerged habit. The leaves of water lilies float on the 

 surface, and therefore are broad ; and being exposed to light 

 are a vivid green, indicating the abundant development of 

 chlorophyll. Many of the pondweeds, however, are com- 

 pletely submerged. As one floats over one of these " pick- 

 erel beds/' the leafy plants may be seen at considerable 

 depths, and have a pallid, translucent look. It will be 

 seen that in these cases the leaf forms are narrow rather 

 than broad, often being ribbon-like, or in some submerged 

 plants even cut up into thread-like forms. It is evident 

 that such narrow leaf forms can respond more easily to 

 water movements than broad forms. The pallid look of 

 these submerged leaves indicates that there has not been 

 an abundant development of chlorophyll. Some pondweeds, 

 however, have both types of leaves, some being submerged 

 and others floating. In these cases it is interesting to notice 



