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. 



(6) 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 

 vulgar e), 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, 



