BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 229 



complicated .arrangement. Several diiferent kinds may 

 spring even from the same leaf (Fig. 38-1). 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 whicli wood 

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

 the older parts of many roots is boujided externally by 

 several layers of cork cells, preventing tlie 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 Ij'ing near tlie wood, and jiush their 

 way through the cortex to the surface. They arise in 

 longitudinal rows. 



XVIII. A BRIEF OUTLINE OF VEGETABLE 

 PHYSIOLOGY 1 



531. Vegetable physiology deals with tlie processes by whicli the 

 life of plants is carried on. Such processes ai-e the absorption of 

 :natevials", the transfer of raw and elaliorated food matters from one 

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

 into organic substance; the storage of elaborated pi'odiicts; 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 hei-baceous parts to but twenty or thirty per cent. 



533. We njay distinguish three ways in whicli water is useful to 

 llie 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 t(jg(ther 

 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 

 I'ather than for ordinary assignment in lessons. 



