168 BOTANY: PRINCIPLES AND PROBLEMS 



with xerophytes and hydrophytes. Their leaf area is extensive, 

 the cuticle and epidermis rather thin, and the stomata not especi- 

 ally protected. Internally they are highly differentiated, partic- 

 ularly as to the fibro-vascular system. 



Through bog and swamp plants, with their typically spongy 

 internal structure, mesophytes merge gradually into hydrophytes ; 

 and at the other environmental extreme it is hard to draw a sharp 

 Hne between mesophytes and xerophytes. Furthermore, there 

 are many plants, such as our deciduous trees, which are meso- 

 phytic during the summer and xerophytic during the winter. 

 Such plants are known as trophophytes. 



The modifications of plants with reference to their water 

 supply are legion and provide a fascinating field of investigation, 

 for nowhere else is the regulatory character of plant structures 

 more clearly evident. 



Chemical Substances. — The effects of chemical substances upon 

 plants are many and far-reaching. In certain cases, particularly 

 in the lower groups, the direction of growth or movement may be 

 determined by chemical stimuli. Far more profound are the 

 structural and functional changes induced by chemical substances 

 entering the plant from the soil. 



We have already enumerated the chemical elements which are 

 essential to the life of plants and have called attention to certain 

 of their specific effects. Iron, for example, has been found to be 

 necessary for the production of chlorophyll. Phosphorus is 

 abundant in seeds and is beheved to stimulate the development of 

 reproductive structures. Potassium seems to have an intimate 

 relation to the process of photosynthesis and an ample supply 

 of this element is necessary if the leaves are to become well 

 developed and to function vigorously. Nitrogen, especially 

 when accompanied by an abundant water supply and active 

 photosynthesis, markedly stimulates the development of vege- 

 tative organs and tends to delay the production of flowers and 

 fruit. These elements and others not only affect the vigor and 

 size of the plant but often the shape and structure of its parts. 



Chemical substances injected into the plant body are also 

 able to stimulate growth in various abnormal ways. Notable 

 instances of this are the great variety of galls produced on plant 

 tissues through insect stings or fungus attacks (Fig. 87). 



Plants display characteristic inherited reactions to the presence 

 of chemical substances in the soil. Certain species, for example. 



