176 PLANT RELATIONS. 



the water, with a few roots dangling from the under side, 

 or with no roots at all (see Fig. 156). This same duck- 

 weed also shows some interesting modifications in its hab- 

 its of reproduction. Although related to plants which pro- 

 duce flowers and make seed, the duckweeds have almost 

 lost the power of producing flowers, and when they do 

 produce them, seeds are very seldom formed. In other 

 words, the ordinary method of reproduction employed 

 by flowering plants has been more or less abandoned. 

 Replacing this method of reproduction is a great power 

 of vegetative propagation. From the disk-like body of 

 the plant other disk -like bodies bud out, and this bud- 

 ding continues until a large group of disks, more or 

 less connected with each other, may be formed. These 

 plants also form what are known as winter buds — well 

 protected bud-like bodies which sink to the bottom of 

 the pond when the floating plants are destroyed, and 

 remain protected by the mucky bottom until the waters 

 become warm again in the next growing season. 



In examining the pond associations, therefore, attention 

 should be paid to the floating forms and the submerged 

 forms, and also to the varying depths of the latter. It 

 will also be noted that the leaves of floating forms are 

 comparatively broad, while those of submerged forms are 

 narrow. 



B. Anchored associations. 



133. Definition. — These are associations fixed to the soil 

 but with submerged or floating leaves. In this case there 

 is still great exposure to water, but there is also a definite 

 soil relation. Two prominent associations are selected 

 from this group for illustration. 



134. Rock associations. — The term rock is used in this 

 connection in a very general way, meaning simply some firm 

 support beneath the water; it is just as likely to be a stick 



