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 w^ell ventilated, or by 

 placing plants in a sheltered place in the open, the leaves 

 have an opportunity of becoming hardened to more 

 rigorous conditions of existence. The outer layer of the 

 young leaves can become strengthened, and they as well 

 as the new leaves will 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 the 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 morejiumid conditions. 



