248 A TEXT-BOOK OF BOTANY. 



It is difficult to separate those factors which act solely as 

 external stimuli from those which are essential to the normal 

 growth of the plant and which may be considered as physiological 

 factors. For example, light under 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 

 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, (b) 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- 

 ment. 



This subject has an important bearing on the physiological 

 testing of drugs. Kobert states that in determining the qualities 

 of a new chemical, preliminary experiments should be conducted 

 on lower plants and animals before trying it on man. Of the 

 plants which have been used in the testing of poisons the follow- 

 ing may be mentioned: Oscillaria, Spirulina, Xostoc, Zygnema, 

 Spirogyra, Saccharomyces, Mucor, Elodea, Lemna, Pistia, 

 Potamogeton, Myriophyllum, Ceratophyllum, Tradescantia, seed- 

 lings of grasses, lupine, bean, pea, corn, etc. Kraemer has em- 

 ployed seedlings of Lupiniis albiis and Pisum sativum in testing 

 solutions containing ethyl alcohol, strychnine nitrate, brucine 

 sulphate, and tincture of nux vomica (Amer. Jour. Phann., 

 IQOO, p. 472; see also Saylor, Ibid., 1916, p. 8). 



Food of Plants. — It has already been pointed out that certain 

 of the chemical elements are necessary for the growth of plants, 

 and that these are derived partly from the surrounding atmos- 

 phere and partly from the soil. Those elements derived from 

 the air are either themselves gases or exist in combination in the 

 form of gas, and include oxygen, nitrogen in exceptional cases, 



