24 
It is sometimes stated that these strange modifications 
are the result of the direct action of the environment on 
vegetation. This is impossible of proof, and we may 
assume that it is largely by natural selection of forms or 
varieties most suited to these extremes of climate that 
desert plants have in the course of ages sprung from 
ordinary types of vegetation. Nevertheless we must not 
forget that to some extent climate has a direct modifying 
action. If, for example, we grow a seedling gorse in a 
moist greenhouse, we find that it will persist for a long 
time in producing small ordinary leaves, which precede 
the spines in the young plant, and this effect is certainly 
a direct action of the surrounding conditions. It is also 
well known that the texture of leaves produced by plants 
grown in a greenhouse differs greatly from that of leaves 
of the same species growing out of doors. It is necessary 
therefore before transplanting into the open plants raised 
under glass, to get them gradually acclimatised to their 
new surroundings. By transferring them from greenhouse 
to frame and keeping the latter well ventilated, or by 
placing plants in a sheltered place in the open, the leaves 
have an opportunity of becoming 4ardened to more 
rigorous conditions of existence. The outer layer of the 
young leaves can become strengthened, and they as well 
as the new leaves wili all be modified to suit the new 
environment. In transferring a plant from a dry to a very 
moist house it will often be noted that ithe older leaves 
are shed. Being fully developed they are unable to adapt 
themselves to the new conditions. Not able to transpire 
freely owing to the moisture-laden atmosphere they become 
overcharged with water and this probably causes them to 
fall. The new leaves will be suitably modified to suit 
the more humid conditions. 
