212 PLANT PHYSIOLOGY 



however, are very considerable, and the student may most profit- 

 ably work up the subject from the literature, which includes a 

 valuable reference and summary in the Botanical Gazette, 45, 

 1908, 286. 



Of the other variable external conditions, the most promi- 

 nent is barometric pressure, which has two forms of fluctuation, 

 viz., daily in correlation with weather changes, and altitudinal 

 with elevation above sea-level. Here again the experimental 

 difficulties will send the student to the literature. Variable chem- 

 ical agents, such as special gases in the air, etc., have already 

 been considered under their effects upon protoplasm. Variable 

 food supply is perhaps of too obvious effect to need experimental 

 study, but its influence can be shown very readily by the simple 

 and well-known method of comparing the growth of three dif- 

 ferently treated sets of seedlings developing from seeds having 

 the food stored in the cotyledons (Beans, Peas, etc.); both cotyle- 

 dons are removed from the plants of one set, one is removed from 

 each of those of another set, and none are removed from the 

 third. To some extent the comparative growth of very large 

 and very small seeds selected from the same stock proves the 

 same fact. 



The consideration of food supply for Growth involves the 

 problem of energy supply, a matter which has already been 

 studied, for growing tissues, under Respiration. The student 

 should now review that work from the present point of view, 

 and extend it to an accurate knowledge of the relations of Respira- 

 tion to Growth. The experimental study, or demonstration, 

 of the dependence of Growth upon Respiration is easy, needing 

 simply some such arrangement as that already described (page 

 126), by which two sets of similar growing tissues are, respec- 

 tively, supplied with, and deprived of, air containing oxygen. 

 Here also the student should inform himself upon the other 

 physical and chemical phenomena accompanying Growth. One 

 physical phenomenon of importance is the power of growing parts 

 to exert much pressure, developed through osmosis, a subject 

 on which some simple experimentation is practicable. 



