154 HETEROPHYLLY [CH. 



The Pontederiaceae 1 and some of the Aponogetons 2 also 

 have band-shaped, submerged leaves in addition to those that are 

 aerial. Scirpus lacustris (Cyperaceae), in 

 which the aerial leaves are very poorly 

 developed, may produce strap-like 

 floating leaves. They were first re- 

 corded by Scheuchzer 3 early in the 

 eighteenth century. 



Some of the Pontederiaceae, e.g. 

 Eichhornia crassipes, present a curious 

 typeofheterophylly the petioles being 

 swollen into bladder-like, floating 

 organs, when the plant grows in its 

 normal free-swimming manner, but 

 becoming slender and elongated when 

 it is thrown upon a muddy shore and 

 takes root there 4 . Experimental work 

 shows that not only a floating life, but 

 full light and low temperature, en- 

 courage the spherical form of petiole, 

 while heat, and poor illumination, tend 

 to reduce it to a more ordinary 

 shape 5 . The bladder-like swellings of 

 the leaves of Pistia also fail to develop 

 when the plant is stranded on mud 6 . 



Examples of heterophylly in aquatics 

 might be multiplied almost without limit, but it is important to 

 remember that they are not unique, and that we often meet with 

 the same phenomenon in terrestrial plants. As Nehemiah Grew 7 



1 Goebd,K. (1891-1893). 



2 Krause, K. and Engler, A. (1906). 



3 Scheuchzerus, J. (1719). 



4 Spruce, R. (1908). 



5 Treviranus, L. C. (1848*) and Boresch, K. (1912). 



6 Hansgirg, A. (1903). 



7 Grew, N. (1682). 



FIG. 103. Potamogeton na- 

 tans, L. Land plant which 

 has been transferred to 

 water and has produced 

 narrow water leaves. (Re- 

 duced.) [Goebel, K. (1891- 

 1893)-] 



