STEMS FUNCTIONING AS LEAVES 117 
flowers, after which they remain dormant till the 
following year. 
It has been seen that unless a plant is a parasite or 
saprophyte, using as food ready-made organic 
material, it is necessary that it should possess a suffi- 
cient expanse of green (i.e., chlorophyll-bearing) 
tissue for the purpose of assimilation. This is the 
essential function of the leaves; but before leaving 
the study of stems it should be pointed out that they 
usually assist, and sometimes entirely replace, the 
leaves as organs of food-manufacture. We have seen 
how in dry places—whether physically dry, from 
direct scarcity of water, or physiologically dry, owing 
to reduced activity on the part of the plant due to 
unfavourable conditions, such as obtain in cold 
regions, or on poisoned ground like salt-marshes or 
bogs—leaf surface tends to be reduced, to avoid ex- 
cessive loss of water. In such plants as the Cacti, 
and the Euphorbias which so closely mimic the cactus 
form, this reduction is carried to its limit. Leaves are 
absent, and the stems, greatly swollen so as to store 
water, take up the process of assimilation, and per- 
form it satisfactorily. In more rapid-growing plants, 
a sufficient area for assimilation may be obtained by 
abundant branching, as in the Gorse, in which leaves 
are present only in the seedling stage. Inthe Brooms 
(Genista) the leaf-development is often weak, but the 
stems sometimes make up for this by bearing green 
flattened wings. In the Spanish Broom (G. sagittalis), 
a straggling shrub inhabiting dry places in south- 
west Europe, the few ovate hairy leaves, produced in 
spring, soon fall; but the slender branches bear 
several (two to four) broad green wings, which act as 
