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 iower 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 
trom 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. ; 
1. The 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- 
nahs, Hippuris vulgaris, Ranun- 
culus delphinifolius, Proserpinaca 
palustris, Roripa americana, ete. 
The epidermis has a thin cuticle, 
or none at all, and is destitute of 
Fig. 37, Floating leaf of Sparganium angus- hairs. The stomata are numer- 
olen. x 130. 
ous 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 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 
