

180 3 FLOWERING PLANTS 
crowded together, free access of air is lessened and _trans- 
piration reduced—e.g., Aloe. 
The effect of light is also well seen by comparing the sees. 
of the same species grown under different conditions. The 
structure of a beech-leaf taken from a tree growing in strong 
light 1s, when examined under the microscope, seen to be 
different from that of a leaf taken from a beech-tree growing 
in the shade. The palisade parenchyma, in the former case, 
is far more-developed, and has more chlorophyll. Warming 
has classified certain trees, according to the light they require, 
as follows : - 
1. Larch, birch, aspen, alder, require the most light. 
2. Scotch fir, ash, oak, elm,.maple, come next. 
3. Pinus montana, spruce fir, lime, hornbeam, beech, require 
least. 3 | 
The development of branches of trees very largely depends 
on how closely the trees are planted, for this affects the 
amount of light that can reach them. In the Scotch High- 
lands, where the Scotch fir abounds, there are no branches on 
the lower part of the trunks of these trees, for enough light 
does not penetrate, owing to the trees being very thick 
together. This also affects the undergrowth. As has already 
been remarked (p. 29), in beech-woods, where the trees -are 
thick there is hardly any undergrowth beyond saprophytic 
plants ; whereas in woods where the trees are less thick, and 
more light penetrates, the undergrowth is considerable. 
TABLE OF COMPARISON OF LEAF STRUCTURES. 

MESOPHYTES. XEROPHYTES. HYDROPHYTES 
(submerged). 
1. Thin cuticle. Thick cuticle; also No cuticle. 
layer of hairs of 
wax for preven- 
tioty ~OFr —-trans- 
piration. : 
2. Numerous’ sto- Stomata deep-_ No stomaia. 
mata, on one. seated. : 
or both sur- 
faces, chiefly on 
the lower. . 
38. Transpiration Transpiration often No transpiration. 
constant. considerably 
lessened. 
