VESTIGIAL AND RUDIMENTARY ORGANS IN PLANTS. 237 
tendril is invariably present. This is probably explicable by 
supposing that both plants sprang from an ancestral type 
which possessed tendrils; but as the two species diverged 
from the ancestral stock, the pea preserved the climbing 
habit and developed it more fully, while the bean abandoned it 
and developed a supporting tissue. Vestigial tendrils are 
found in many leguminous plants, such as Lathyrus pratensis. 
The interchange of structure and function between leaf and 
stipule is best seen in Lathyrus Aphaca. In this plant the 
stipules are large and leaf-like and function as leaves, while 
the pinne of the leaf have been wholly transformed into long 
tendrils. In the furze (Ulex ewropeus) true leaves are only 
found on the young plant, and on young shoots, and it is 
interesting to trace the evolution of spines from these leaves 
as the plant grows, because they are the organs correspond- 
ing to leaves which the furze possesses in the adult state. 
Spines, in this case, are vestigial leaves developed as protec- 
tive organs. The green assimilative tissue, which should have 
formed the leaf, surrounds the spines and younger twigs, and 
thus the furze may be described as an evergreen without 
leaves. This characteristic feature is also seen in the broom, 
but here small leaves are produced as well, which are shed 
in autumn, while shoots which bore them remain green 
throughout winter. This assumption by shoots and twigs of 
the functions of leaves, and the partial or complete atrophy 
of the latter, are directly traceable to external conditions. 
Where these conditions are extreme, the prevailing foliage 
becomes abnormal. The flora of a humid swamp in a 
tropical land is characterised by the large size of the leaves 
of its individual members, inasmuch as heat, light, and 
moisture are abundant, while in such localities respiration 
cannot be as active as it is in drier places. Hence a large 
transpiring surface is necessary, and this end is obtained by 
the greater size of the leaves. The opposite extreme—that 
in which respiration is at a maximum, and where rain 
scarcely or never falls—is illustrated by the xerophilous 
floras of arid tropical uplands, such as that of the Mexican 
plateau. The prevailing feature of such a flora is the 
abundance of species of Cactus, Opuntia, and Euphorbia, in 
which the leaves are either vestigial or have been wholly 
