60 BRITISH PLANTS 



the physiological winter — e.g , Gerastium semidecandrum, 

 Trifolium arvense, Aira- 'prcecox, JBromus mollis, Phleum 

 arenarium, Jasione montana, and Draba verna (see p. 107). 



(6) Weeds of Cultivation in dry fields and waste places 

 — e.g., cudweed (woolly), Lepidium ruderale (leaves small, 

 plant shrubby), corn-spurrey (fleshy), Saxifraga tridactyl- 

 ites (rosette-form), etc. 



3. Mesophytic Annuals — e.g., corn-buttercup, corn- 

 cockle, herb-Robert, black medick, fool's-parsley, cleavers 

 (a climber), yellow rattle, Poa annua, etc. 



Biennials. — Most biennials at the end of the first season 

 die down to tuberous underground structures (p. 111). 

 These often carry a rosette of radical leaves close to the 

 ground (see rosette-type, p. 37) — e.g., foxglove, carrot, 

 burdock, ox-tongue, etc. 



Deciduous Trees and Shrubs. — At the approach of winter, 

 these plants lose their leaves. They are not destroyed by 

 cold or stripped off by the wind, but are discarded, as it 

 were, by an act of the tree itself. The deciduous leaf is 

 essentially a summer leaf, typically large, thin, hori- 

 zontally placed, exposed freely to the light and the wind, 

 and richly provided with stomata on its under surface. 

 Although these characters favour transpiration, there is 

 little danger of excessive transpiration in summer, because 

 the soil is warm and the supply of water is adequate. In 

 winter, however, the possession of these leaves would be 

 injurious, and in most cases fatal to the plant. The soil 

 is cold, and very little water is absorbed by the roots. 

 If, under these circumstances, transpiration could not be 

 controlled, the plant would lose more water than it could 

 get, and death would follow from drought. To meet this 

 peril, the tree forms a layer of cork across the base of the 

 leaves in autumn. The leaves, thus cut off from their 

 water-supplies, dry up and die, and, a layer of cells just 

 outside the cork splitting, they easily become detached 

 and fall to the ground. And so the tree, which during 

 the summer stood up, with its thousands of expanded 

 leaves — a typical mesophyte — becomes now a leafless 

 xerophyte, with bare cork-covered twigs, and the buds 

 which win renew its foliage in the spring protected from 

 desiccation in many and wonderful ways. 



Buds and Bud-Protection. — ^In perennial plants, the 

 assimilating organs are formed in buds, which are situated 



