128 



THE PLANT 



stantly below the water ; in amphibious ones, they grow normally in the air. 

 Floating plants have leaves in which the upper surface is in contact with the 

 air, and the lower in contact with the water. Transpiration is at a maximum 

 in the amphibious plant ; it is reduced by half in the floating type, and is 

 altogether absent in submerged plants. Aeration reaches a high develop- 

 ment in amphibious and floating forms, but air-passages are normally absent 

 from submerged forms except as vestiges. Photosynthesis is marked in the 

 former, but considerably weakened in the latter. The vascular system, 

 which attains a moderate development in the amphibious type, is considerably 

 reduced in floating forms, and it is little more than vestigiate in 



submerged ones. 



I. TJie amphibious type. 

 Plants of this type grow in wet 

 soil or in shallow water. The 

 leaves are usually large and en- 

 tire, the stem well developed, and 

 the roots numerous and spread- 

 ing. In the majority of cases the 

 leaves are constantly above the 

 water, but in some species the 

 lower leaves are often covered, 

 normally, or by a rise in level, 

 and they take the form or struc- 

 ture of submerged leaves. This 

 is illustrated by Callitriche autum- 

 nalis, Hippuris vulgaris, Ranun- 

 culus delphinifolius, Proserpinaca 

 paliistris, Roripa americana, etc. 

 The epidermis has a thin cuticle, 

 or none at all, and is destitute of 

 hairs. The stomata are numer- 

 ous and usually more abundant 

 The palisade tissue is represented 



Fig. 37. 



Floating leaf of Spanraniutn angus- 

 tifolium. X 130. 



on the upper than on the lower surface 

 by one or more well-developed rows, but this portion of the leaf is regularly 

 thinner than that of the sponge part. The latter contains large air-pas- 

 sages, or, in the majority of cases, numerous air-chambers, usually provided 

 with diaphragms. The stems are often palisaded, and are characterized by 

 longitudinal air-chambers crossed by frequent diaphragms, which extend 

 downward through the roots. 



2. The floating type. With respect to form and the structure of the upper 

 part of the leaf, floating leaves are essentially similar to those of amphibious 

 plants. They are usually lacquered or coated with wax to prevent the 



