LONGEVITY OF PLANTS 109 
deciduous tree, for example, fall at the end of summer, 
and must, therefore, be renewed annually. Perennials 
may be classified according to the longevity of their 
assimilating parts : 
(a) Evergreen Perennials.—The leaves of these plants 
live through the winter ; they may last several years, but 
sooner or later their vitality fails, and they drop. They 
do not, however, all fall off together, and the plants are 
never leafless. Winter-green plants, if land-plants, are 
all xerophytes outside the Tropics. They are character- 
istic of climates which are hot and dry during summer— 
e.g., the Mediterranean laurels and myrtles; where 
summer and winter are both inhospitable seasons— 
e.g., the coniferous forest ; or where the winter season is 
very long—e.g., alpines. 
(b) Deciduous Plants.—In these certain of the vegetative 
organs, always including the leaves, are shed annually : 
(i.) Deciduous Trees and Shrubs, in which the shoots 
are all long-lived, and only the summer-green foliage is 
annually shed. They fall simultaneously at the approach 
of winter, leaving the trees bare. In some coppiced 
plants, however, the leaves may not all fall together. 
The oak, for example, when pruned in hedges, forms the 
usual plate of cork across the base of the leaves at the 
end of summer, the leaves die, but the final rejectment 
by the splitting of the separation-layer is not com- 
pleted, so that many dead leaves remain on the tree 
during a large part of the winter—falling gradually, as 
they are blown off. A still more familiar instance of 
pruning interfering with leaf-fall is the privet (Ligustrum 
vulgare), planted everywhere in gardens and hedges. When 
kept well pruned, many of the leaves remain alive and 
green upon the bushes till the new leaves break from the 
buds in spring. Pruning is an artificial way of reducing 
transpiration, and acts, therefore, as a xerophytic 
adaptation. 
(ii.) In other plants the smaller twigs are shed annually 
as well as the leaves. 
(iii.) In others the leaves die and all the shoots except 
those close to the ground. These bear the renewal-buds, 
and in many cases leaves as well—e.g., chrysanthemum. 
(iv.) Herbaceous Perennials.—Here the destruction of 
the aerial parts is complete. The individuals, however, 
