106 BOTANY. [CHAP. iu. 



for fulfilling their functions ; but in the cycads, screw- 

 pines, and palms, the leaves still retained the densely 

 crowded rosette arrangement, whereas in Dicotyledons 

 this difficulty was removed by lengthening the stem 

 between the individual leaves, which resulted in their 

 being scattered at distant intervals on the twig, as in 

 the ash, oak, rose, etc. 



Plants belonging to cold regions are characterized 

 by having the leaves covered with a dense felt of soft 

 interwoven hairs for purposes of warmth. When such 

 plants are acclimatized to warmer regions, this protec- 

 tion in many instances is much reduced or completely 

 disappears. 



The protection of the individual by various modifica- 

 tions that enables a given amount of leaf surface to do 

 the greatest possible amount of work, is very clearly 

 shown in the evolution of the stem or trunk. In Mono- 

 cotyledons, as already explained, the trunk is usually 

 unbranched, and bears at the top a dense rosette of large 

 leaves as in the palms ; one or two palms and the screw- 

 pines are exceptional in having a branched stem, but the 

 leaf arrangement is the same. In Dicotyledons we find 

 the main trunk broken up into numerous spreading 

 branches bearing many small leaves scattered at inter- 

 vals on the twigs, so that practically every inch of leaf 

 surface is fully exposed to the light, and consequently 

 there is no manufacture of leaf material that when made 

 cannot fully perform its functions on account of imper- 

 fect exposure to light. From geological evidence, the 

 earliest type of stem for the purposes of removing the 

 leaves beyond the reach of herbivorous animals, and at 

 the same time exposing them to light and air, was the 



