STEMS AND THEIR FUNCTIONS 165 



The transporting system of the plant, on the other hand, is composed 

 of two independent units, each doing a different job and operating on a 

 different principle and neither of them at all like the circulatory system 

 of the animal. The xylem conduits form a unidirectional system of water 

 transport, operating largely through transpiration pull — a mechanical 

 process. The phloem channels are made up of the linked protoplasm 

 of the phloem cells and through them, in either direction, move food and 

 inorganic salts by diffusion or cell activity. Neither part of the vascular 

 system is vitally concerned with respiration or excretion; oxygen and 

 carbon dioxide are carried in solution in both, but the supply and elimina- 

 tion of these gases is accomplished for the most part by direct interchange 

 with the atmosphere. 



In addition to the primary functions of support and transportation, 

 parts of the stem (particularly the phloem and parenchyma) may serve 

 for temporary storage of manufactured food, water, or other substances. 

 In many plants, also, the stem may play an important part in vegetative 

 reproduction. 



