182 



FRESH-WATER BIOLOGY 



thread-like leaves are very common and may be the only kind occur- 

 ring on the plant, as in Potamogeton pectinatus, or they may occur 

 on the same plant together with floating 

 leaves, as in Potamogeton natans (Fig. 255). 

 It is to be noticed that most of the monocoty- 

 ledons conform to some one of the types 

 mentioned, while the dicotyledons seem to 

 favor another habit, such as is seen in the 

 finely dissected leaves of Ranunculus aquatilis, 

 Myriophyllum spicatum, Bidens beckii (Fig. 

 256), and Ceratophyllum. Among the dicoty- 

 ledons in which both floating and submerged 

 leaves are present, as in Ra- 

 nunculus and Cabomba (Fig. 

 257), the tendency to finely 



dissected leaves is conspicuous, while in the monocot- 

 yledons, having both floating and submerged leaves 

 on the same plant, the latter tend to assume the 

 ribbon-like or the long linear outline, as in Fig. 255. 

 Some of the true water plants, as Bidens beckii and 



Myriophyllum spi- 

 catum, support a 

 vertical portion of 

 the main stem con- 



FIG. 255. Potamogeton natans. 

 One floating leaf and three 

 submerged leaves, representing 

 the thread-like form of the 

 monocotyledonous type of sub- 

 merged leaf. (After Gobel.) 



siderably above the 



finely dissected. 

 Emersed leaves en- 

 tire or slightly ser- 

 rate. One whorl 

 shows the transition 

 stage from the sub- 

 merged to the 

 emersed form. $ 

 natural size. 

 (After Gobel.) 



water surface and 

 on this emersed 

 portion ordinary 

 aerial leaves are 

 borne. It is some- 

 times possible in the case of such 

 plants to find leaves which seem 

 to be midway in form between the finely cut submerged leaves and 

 the bladed emersed ones, so it seems probable that the submerged 

 leaves are to be regarded as exposed leaves which have changed in 

 form because life under water requires such modification. Such a 

 modification has been produced experimentally. Some plants in 



FIG. 257. Cabomba. Floating leaves, entire and pel- 

 tate. Submerged leaves with finely dissected 

 blades typical of Dicotyledons. (After Gobel.) 



