44 



BRITISH PLANTS 



of acacia, Fig. 13), or by slioots which function as leaves. 



These shoots are of two kinds : 



(a) The leaves are small and green or reduced to scales, 



and all the stems take over the functions performed by 



leaves, as in switch- 

 plants — e.g., horsetail, 

 broom (Fig. 14). 



(6) The leaves are 

 again reduced to scales, 

 but most of the 

 branches are flattened, 

 and resemble ordinary 

 foliage - leaves — • e.g., 

 butcher's - broom 

 (Fig. 15). These flat 

 branches, known as 

 cladodes or phyllo- 

 clades, are at once 

 distinguished from true 

 leaves by their position 

 in the axils of the 

 scales, and because 

 they themselves often 

 bear scales, and even 

 flowers, on their upper 

 surface or along their 

 edges. Cladodes are 

 distinguished from 

 phyllodes in the same 

 way, for, after all, the 

 latter are only parts of 

 leaves, while cladodes 

 are shoots. 



7. Lack of nutrition 

 sometimes reduces 

 shoots to thorns, and 

 leaves partially or en- 

 tirely to spines. Thorny 

 plants are pronounced xerophytes, and form a con- 

 siderable part of the bush and scrub vegetation of semi- 

 deserts. In England, gorse is characteristic of dry 

 heaths. Some plants even have two forms ; Ononis arvensis 

 (rest-harrow) growing on the seashore usually develops 



Fia. 13. — SsBDLiNa ov Acacia mdano- 

 xylon, SHOWING Tbansition feom 

 Ordinaby Petiole (6) to Phyllode (d). 

 (About Halt Natural Size.) 



In c the petiole is slightly winged. 

 a, cotyledons ; e, root-nodules. 



