ADAPTATION TO WATER 167 



majority of cases the leaves are constantly above the water. The 

 lower leaves of some species are covered, either normally or by a 

 rise in level, and take the form and structure of submerged leaves. 

 In Callitriche autumnalis and Hippuris vulgaris, the submerged 

 leaves show changes of size and structure, while in the heterophyl- 

 lous species, Ranunculus delphinifolius, Proserpinaca palustris, 

 Roripa americana, etc., they differ from the aerial leaves in being 

 greatly dissected. 



The leaves of amphibious plants, with the exception of those 

 just mentioned, are usually large and entire, the stem well developed, 

 and the roots numerous and spreading. The epidermis has a thin 

 cuticle or none at all, and is destitute of hairs. The stomata are 

 numerous and usually more abundant on the upper than on the 

 lower surface. The palisade tissue is represented by one or more 

 well-developed rows, but this portion of the leaf is regularly thinner 

 than the sponge part. The sponge tissue contains large air 

 passages, or numerous large air chambers, usually provided with 

 thin plates or diaphragms of cells. The stems are often palisaded, 

 and are provided with longitudinal air chambers crossed by fre- 

 quent diaphragms. 



Experiment 52. Structure of amphibious plants. Draw in outline a 

 representative amphibious plant, such as Alisma, Ranunculus, or Sagit- 

 taria. Outline an aerial and a submerged leaf of Callitriche or Hippuris, 

 and of a heterophyllous species, such as Ranunculus delphinifolius. Draw 

 a segment from a cross-section of the aerial and submerged leaves of 

 one of the foregoing. 



186. Floating plants. In the form and structure of the upper 

 portion, floating leaves are essentially similar to those of am- 

 phibious plants. They are usually coated with wax to pre^ent 

 the clogging of the stomata by water. Stomata are found only 

 on the upper surface, with the exception of a few cases where the}' 

 persist with loss of function upon the lower side. The palisade 

 tissue of the leaf is much less developed than the sponge tissue, 

 which is filled with enormous air chambers. The stems or the 

 petioles are much elongated, and the aerating system is greatly 

 developed, while the supportive tissues, i.e.. the fibroA'ascuIar 

 bundles, are reduced. In the Lemnacea;, the leaf and stem are 

 represented by a tiny thallus. The roots are in the i)rocess of 

 disap]iearing; for examjjle, Spirodela has several, Lemna one, ami 

 Wolffia none. 



