1 86 THE BIOLOGY OF FLOWERING PLANTS 



may be permanent or temporary. A temporary reduction 

 is shown by all our deciduous trees and shrubs, and by 

 perennial herbs which die down in autumn, persisting as 

 underground roots, rhizomes, bulbs, or corms. This fall 

 of the leaf is not related to transpiration only ; protec- 

 tion against frost is also important. Yet if we think of the 

 leaf structure of the evergreen trees and shrubs we find 

 that it shows the features we associate with reduced tran- 

 spiration. The glistening leaves of ivy and holly indicate a 

 thick cuticle ; the heaths show protection of stomata, as do 

 the evergreen conifers. It is interesting to compare with 

 the latter the needles of the deciduous larch in which the 

 cuticular development is small and the stomata are flush 

 with the surface. 



Transpiration is, of course, normally low in winter ; 

 but trees in foliage through this period run a danger when, 

 in periods of temporary high transpiration — warm sunny 

 days — the water supply from a cold soil, or through a frozen 

 trunk, is very slow. The sun is often brilliant and may 

 radiate considerable heat in frosty weather. This danger 

 is real. A conifer is often seen with the needles on one side 

 dead, brown, and burnt ; this is generally regarded as due, 

 not to the direct effect of the heat or light of the sun, nor to 

 " frosting," but to transpiration so high, under conditions 

 of insufficient water supply, that the leaves are actually 

 desiccated. Neger (191 5) considers that it is due to the 

 action of late frost on young needles. But it is certain that 

 winter leaf-fall is partially related to the danger of excessive 

 transpiration. 



A much more definite relation occurs in those plants 

 in which the leaves die during the hot season. This is 

 a characteristic feature of the bulbous plants of arid regions. 

 In the Mediterranean countries and in South Africa the 

 bulbous plants sprout in the early spring or in the wet 

 winter season, complete their flowering, and die down as the 

 dry season commences. The bulbous plant is, indeed, 

 characteristic of semi-arid regions. Our few representatives 

 may have been derived from such types. Their vernal 



