FHE LEAP 17 
4, The floating type. 
On the whole, the structure of the floating leaf approximates 
to that of Mesophytes, for the upper surface has a cuticle, 
which is often waxy ; the palisade tissue and stomata are 
also present. Speaking generally, the leaves of Water-Plants 
have no protection against too rapid transpiration. They have 
enormous intercellular spaces full of air, and the leaf-stalks 
often act as floats. 3 
Leaves that are entirely submerged have no cuticle and no 
stomata, for they never transpire ; they take in water from 
their whole surface. 
It has been shown how the minute structure of the leaf 
enables the plant to adapt itself to the different conditions of - 
life which obtain in various regions. It may now be con- 
sidered how the form of the leaf as a whole, and its arrange- 
ment on the stem, are connected with the habitat of the plant. 
Influence of | Lhe physical factor which more than any other 
ae nie determines the form and general arrangement of 
Arrangement the leaf is light. ‘Io estimate this, the leaves 
of Leaves. of tropical plants pay be compared with those of 
temperate regions. 
l. Tropical plants, speaking very generally, have leaves 
Standing straight up, so that the light falls on the edge, not on 
the surface, of the blade. This is seen in the Acacia (Plate V., 
Frontispiece).. In this way tropical plants protect themselves 
from the intensity of the light. On the other hand, in 
temperate climates, especially where the summers are short 
and often wet, plants want as much light as they can get. 
They spread out their leaves in such a way that the surface 
of the blade shall be as much exposed to light as possible, and 
the arrangement of the leaves on the stem is such that one 
does not shield the light from another. | 
2. The leaves. of tropical plants are often linear, and 
small, as in the Oleander, Eucalyptus, whilst those of tem- 
perate regions are big and broad. Tropical plants often have 
tolded leaves, as in the Grasses and Palms, whilst those of 
Plants of temperate regions are flat and smooth. Some 
tropical plants have leaves arranged in rosettes; being thus 
