266 A TEXTBOOK OF BOTANY [Ch. V, 10 



III. Functions performed not in special tissues but in special 

 regions of the tissue systems. 



9. Storage of food and water. No special position is 

 requisite ; hence it occurs in any available regions of pith, 

 cortex, medullary rays, and other parts, which often, by 

 multiplication of the cells, produce swollen tubers, bulbs, etc. 



10. Secretion and excretion. No special position is 

 requisite, but usually occurs in special cells, as the idio- 

 blasts, or collections of cells forming glands, resin pas- 

 sages, etc. The latex system includes tubes ramifying 

 throughout the other tissues (page 134). 



11. Dissemination. Involves no special tissues, but 

 modifications, with special outgrowths of hairs, wings, 

 hygroscopic walls, etc., of superficial tissues, epidermis, 

 and cortex, as described in a later section. 



IV. Functions performed in all living cells. 



12. Metabolism, or chemical changes (apart from photo- 

 synthesis and respiration) involved in the life processes. 



13. Respiration, or release of energy by oxidation. Re- 

 quires usually the access of free oxygen, which is effected 

 by development of an intercellular air system (aeration 

 system) ramifying everywhere throughout the plant, and 

 opening to the atmosphere through stomata or lenticels. 

 There occurs in some cases a special development of tissues 

 with large air passages, called aerenchyma. 



14. Self- adjustment to immediate surroundings. Re- 

 quires perception of the external stimulus, transmission to 

 a motor zone, and a motor response mechanism. For all 

 of these phases special structural features have been de- 

 scribed, usually special modifications of tissues, as in case 

 of the starch sheath (page 130). 



