266 THE EFFECT OF MOISTURE 



aerial. As regards the leaves, it is well known that 

 when aerial and floating leaves are present on the same 

 aquatic plant, they differ greatly in structure, and as a 

 rule also in form, from the submerged leaves. In 

 Ranunculus heteropJiyllus and Cabomba aquatica, for 

 instance, the floating leaves are more or less rounded, 

 whilst the submerged ones have dissected and filiform 

 segments. In Hippuris (Mare's tail) the aerial and 

 floating leaves are short, and in CallitricJie rounded, 

 but the submerged leaves of both are linear or ribbon- 

 like. In all cases the submerged leaves are of a 

 more delicate texture, more or less translucent, and of 

 a brighter green colour than the others. They show 

 degradation of anatomical structure in every part, the 

 cuticle and stomata disappearing, whilst the chlorophyll 

 grains and the mesophyll are greatly reduced in quan- 

 tity.* Even better evidence of the direct relation be- 

 tween environment and character is afforded by certain 

 other observations of Costantin. Thus he found that 

 he could change the form of Hippuris at will, " by 

 transplanting an aquatic plant on to land, and vice 

 versa; all the leaves produced under water were long, 

 undulated, and delicate; whereas those in air were short, 

 erect, and firm." Again, he found that the leaves of 

 Sagittaria (Arrowhead), when deeply submerged, are 

 soft and flexible, and may reach a length of over six 

 feet, but when developed in air they are short and 

 erect. 



When a leaf is full grown, sudden change of environ- 

 ment kills it, aerial leaves perishing under water, and 



*F^Henslow's "Origin of Plant Structures," chap, viii., from 

 which the greater part of this paragraph is drawn. 



