38 THE STORY OF PLANT LIFE 



So far as the stem is concerned the chief signifi- 

 cance to the plant is the manner in which it exposes 

 the rest of the organs, the leaves, flowers, and fruit 

 to the agencies which specially affect each, the air 

 and light in the first case, insects, etc., in the two 

 latter. 



The stem also differs markedly in its duration, 

 extent of wood formation, the arrangement of the 

 branches, and its coverings. Some stems are pro- 

 tected from animals by thorns or prickles, e.g. 

 Holly, Hawthorn. The glands serve also for excre- 

 tion. The stiff hairs form a chevaux de frisc against 

 the ascent of creeping insects. The stems of plants 

 are also adapted in relation to the distribution of 

 the leaves, and the character of the leafstalk to the 

 carriage of water to the root, or to the exterior of the 

 plant, where the branches and leaves form a close 

 canopy and water drips around the circle so formed 

 and at a distance from the primary root or axis. 



The chief physiological function of the leaf is the 

 securing of food-materials for nutrition, from the 

 atmosphere, by photosynthesis or assimilation. The 

 leaves also take part in the processes of respiration 

 and transpiration. They are variously adapted for 

 these purposes in different types of plants, e.g. in 

 water plants as opposed to land plants, or according 

 as the habitat requires of the plant different modes 

 of response to the conditions of existence. 



The arrangement of the leaves upon the stem is 

 connected with these functions. They are arranged, 



