1890.] of South African Plants to the Climate, 38 



shrubs made upon me on my arrival here. It was the middle of 

 summer, they were then bare, utterly devoid of leaves or fruit, and 



to all appearance quite dead ; but, a few months later, they presented 

 a very different appearance with their green leaves and red berries, 

 This was the first observation which demonstrated to me the fact that, 

 at least in this part of South Africa, the season of rest for the veget* 

 ation is not the winter, but the summer. 



Thus far those plants have been discussed which avoid the hardships 

 of the dry season by a judicious retreat ; let us now turn to those which 

 face the enemy bravely. 



Before doing so, however, allow me to remind you of a few facts 

 connected with the physiology of plants. The organs that absorb the 

 water are the roots. Thence it passes into the stem, branches and 

 leaves. The leaves and other green parts of plants are the organs 

 which assimilate, that is to say, which prepare organic substance from 

 the carbonic acid of the air, the water and other materials taken up by 

 the roots. They inhale the carbonic acid and exhale the oxygen 

 through the stomata, the so-called breathing pores. This latter name 

 is not well chosen, and I think that it is partly responsible for the 

 popular idea, according to which the leaves are considered as the lungs 

 of the plant, while they are in reality its stomach and organs of 

 digestion. 



Quite independent of assimilation is respiration. The breathing process 

 goes on continually not only in the leaves, but throughout the whole 

 plant. During the exchange of gases liberated by these processes 

 some water also necessarily evaporates. Little or nothing of this 

 vapour can be lost during its passage through the stern,, but during the 

 necessary ventilation through the proper outlets, the stomata, the 

 water vapour also escapes ; besides this, if not prevented by special 

 means, there is also the evaporation of water through the epidermis of 

 the leaves. Hence it is necessary for the leaf to regulate its rate of 

 evaporation according to the supply and to reduce it to a minimum 

 during the dry season. 



Plants not exposed to very trying conditions are enabled to do this 

 by the automatically -acting mechanism of the stomata, for these little 

 slits close when the tissue of the leaf loses too much water, and open 

 again when the supply is sufficient. With plants, however, of dry 

 localities, or plants which have to last through periods of drought, 

 other and more efficient means to check the loss of water or to secure 

 a sufficient supply are necessary. 



The variety of arrangements for this purpose is very great. In the 

 following description they are arranged in seven groups. I need 

 hardly add that the formation of these groups is quite arbitrary and 

 simply a matter of convenience. 



