DISSECTED LEAVES 123 
doubted advantage as allowing light to pass through 
to lower layers of leaves; it also materially diminishes 
the danger arising from excessive wind-pressure. In 
the former case there is often a wide space 
between the divisions of the leaf; but where this is 
not required, the parts of the leaf fit closely together, 
to secure a maximum of surface. A particularly 
pretty example is seen in the Chilian shrub Wein- 
mannia trichosperma (Fig. 22). Here, to avoid the 
loss of the area between the leaflets, the mid rib steps 
in, developing triangular wings which fill the spaces. 
It might be objected that the plant might have saved 
itself much trouble by producing, while it was about 
it, a simple undivided leaf covering the whole area. 
It is difficult to answer such suggestions. Probably 
the present form of the leaf best meets the conditions 
of wind, rain, and light under which it lives. Pos- 
sibly its present form is bound up with its ancestral 
history. “It must be acknowledged,” says D. H. 
Scott, “that nothing is more difficult than to find out 
why one plant equips itself for the struggle with one 
device and another attains the same end in quite a 
different way.” 
During cold and tempestuous weather the presence 
of leaves may be a danger to the plant rather than a 
help; and where seasonal variations are such that 
strongly contrasted periods of favourable and un- 
favourable weather occur, such as the summer 
and winter of our own climate, many plants have 
adopted the device of shedding all their leaves: this 
is especially characteristic of the largest plants (the 
trees), which would naturally suffer most from un- 
favourable weather. The fall of the leaf is accom- 
