66 
EVERGREEN AND DECIDUOUS TREES, ETC. 
of evergreen forms. But from this period upwards the strata 
deposited in the temperate zone contain an increasing pro¬ 
portion of deciduous exogens, mixed with a few of the earlier 
evergreen forms, and the still persistent and evergreen Coni¬ 
fer a. 
The association of the deciduous habit with extreme 
variation of seasons is not difficult to explain. Leaves are 
the organs by which the functions of vegetable life are chiefly 
carried on. They are the principal organs of respiration, 
transpiration, assimilation, and secretion. By means of their 
form and position, large surfaces of sensitive tissue are 
brought into direct contact with air and light, but the neces¬ 
sary consequence is that they are, more than any other parts 
of the plant, liable to injury by violent changes of tempera¬ 
ture, and especially by frost. 
In regions whose mean annual temperature is above 55° 
snow does not lie and ice is rarely seen. The functions of 
vegetable life can be carried on throughout the year, although 
with diminished energy during the colder or drier periods, 
and the leaves of plants remain green and active until new 
ones take their place. But a mean temperature below 50° 
implies great depression in the winter months, and frequently 
severe frost such as the leaves of most plants are not able to 
bear. If they are killed while the cells are full of active 
chlorophyll, which has been providing daily food for the 
growth of the plant but has laid up no store, there will be 
nothing to support the fresh growth of spring, and the plant 
must dwindle or die. It would seem that by the law 
of natural selection and survival of the fittest, plants which 
were developed in climates gradually becoming more varied 
and more severe must have adapted themselves to these 
conditions by converting their chlorophyll into starch and 
storing it in special cells at the base of the buds as soon as 
the cold and comparative darkness of autumn began to check 
the vital functions. The leaves exhausted of chlorophyll 
would be of no further use and would be cast off, while the 
plant passed into a state of hibernation, withdrawing its 
remaining chlorophyll even from the surface of the young 
twigs, that it might be preserved for the time of re-awakening. 
This process is plainly analogous to the hibernation of animals 
in similar climates. Fattened by the abundant food obtainable 
in autumn, they are able to sleep through the season of cold 
and darkness, and to wake with a sufficient reserve to carry 
them on till fresh food is found. Deciduous plants are those 
which hibernate; and as some animals maintain themselves 
through the winter while others have acquired the habit of 
