BEIEF OUTLINE OF VEGETABLE PHYSIOLOGY 229 



complicated arrangement. Several different kinds may 

 spring even from the same leaf (Fig. 384). Stinging 

 hairs (Fig. 360) and hairs with bitter secretions are an 

 important means of defense to many plants. 



530. The anatomy of the root resembles, in a general 

 way, that of the stem. There is a central conducting 

 and strengthening strand of wood. In the older roots of 

 perennial exogenous plants this becomes a cylinder of 

 wood surrounded by a cambium zone, from which wood 

 is formed annually just as in the stem. The cortex of 

 the older parts of many roots is bounded externally by 

 several layers of cork cells, preventing the passage of 

 water into or out of the root. Formation of new tissue 

 for growth in length takes place at the growing point 

 (Fig. 28, g) under the root cap. New lateral roots 

 originate from cells lying near the wood, and push their 

 way through the cortex to the surface. They arise in 

 longitudinal rows. 



XVIII. A BRIEF OUTLINE OP VEGETABLE 

 PHYSIOLOGY 1 



531. Vegetable physiology deals with the processes by which the 

 life of plants is carried on. Such processes are the absorption of 

 materials ; the transfer of raw and elaborated food matters from one 

 part of the plant body to another ; the conversion of inorganic matters 

 into organic substance ; the storage of elaborated products ; respira- 

 tion and the consumption of food for the production' of vital energy ; 

 growth ; and movement. 



532. Constituents of the plant body. The chief constituent, as 

 concerns quantity, is water, since even in woody parts the solid por- 

 tions amount at most only to fifty per cent of the total weight, and 

 in herbaceous parts to but twenty or thirty per cent. 



533. We may distinguish three ways in which water is useful to 

 the plant: (1) it furnishes part of the raw material out of which 



1 A number of experiments in vegetable physiology and some informa- 

 tion as to the general function of plants have already been given in this 

 book. The present chapter is added for the purpose of gathering together 

 in coherent form the results of these previous studies. As discussions of 

 the most important matters will be held in the class room, following 

 experimentation in the laboratory, the chapter may be used for reference 

 rather than for ordinary assignment in lessons. 



