4 BOTANY AND PHARMACOGNOSY. 



plant, as potassium, magnesium, phosphorus, sulphur, iron and 

 calcium. The latter element does not seem to be necessary to 

 the normal development of some of the Fungi and certain Algae. 



Water permeates all parts of the plant and when the cells 

 are in the normal turgescent state it contains more than half 

 its weight of water. When the supply of water falls below the 

 normal the plants begin to droop and finally die. The need of 

 plants varies greatly in this particular ; some are aquatic in their 

 habits and live wholly in the water; others can live only on the 

 land ; and still others are adapted to desert regions. 



The degree of temperature necessary for growth varies within 

 certain limits for each kind of plant, but as is stated by Pfeffer, 

 the greatest extremes are shown by Fungi, Bacteria and the 

 lower Algse. Generally speaking the most favorable temperature 

 for growth is between 24 and 34 C. 



Besides the factors enumerated there are other factors which 

 influence growth. They include light (p. 106), gravity (p. 94), 

 mechanical agencies, etc., and are sometimes spoken of as external 

 stimuli. 



It is difficult to separate those factors which act solely as exter- 

 nal stimuli from those which are essential to the normal growth 

 of the plant and which may be considered as physiological fac- 

 tors. For example, light vmder certain conditions may be 

 regarded as in the nature of an external stimulus and not essen- 

 tial to the growth of the plant, while in other cases it has a direct 

 influence on normal growth and is essential to the life of the 

 plant, as in all plants or parts of plants where photosynthesis 

 (p. 109) takes place. 



In addition to the essential food elements, there are many 

 substances which affect the growth of plants which may be 

 grouped as chemical stimuli, such as (a) the substances secreted 

 by gall-forming insects, (h) in a certain measure some of the 

 substances produced by Fungi, (c) and numerous substances not 

 found as normal constituents of the plant. Depending upon the 

 amount of the substance present and the conditions under which 

 it is supplied, the substance may act as a poison and injure the 

 plant, or it may accelerate growth, or cause abnormal develop- 

 ments. 



