Volume 12 Number 3 



The Plant World 



A Magazine of General Botany 

 MARCH, 1909 



ROLES OF THE SOIL IN LIMITING PLANT ACTIVITIES. 



By Burton Edward Livingston. 



According to modern physiological philosophy, the life of 

 a plant is to be considered as a series of reactions on the part 

 of the organism to the ever changing conditions both within 

 and without its body. These reactions, which make up the 

 life activities of the plant, can take place only within certain 

 limits, they can occur only when external conditions are so 

 arranged that they bring forth reactions in the right direction 

 and of the right intensities. Thus a mature seed, in which the 

 internal conditions have not been disarranged since it left the 

 parent plant, will react bv germinating when the temperature 

 and moisture conditions lie within certain limits, these limits 

 varying considerably with the kind of seed involved. Another 

 condition limiting germination is the rate of supply of oxygen. 

 Germination fails to occur if the temperature is either too high 

 or too low, and the same lack of response is noted with too low 

 a rate of moisture or of oxygen supply. The plant continues 

 to grow as long as water, salts and oxygen can reach the growing 

 tissues as rapidly as they are fixed by growth or lost by outward 

 diffusion, and as long as the temperature neither falls below 

 nor rises above certain limits. Also, in the case of green plants, 

 growth can continue only with the influx cf certain kinds of 

 light energy, at a certain rate, and with the inward diffusion of 

 carbon dioxide at a velocity great enough to supply the green 

 tissues with material for elaboration. If the external conditions 

 change at any time, a more or less marked effect is produced 

 upon the plant, and if such a change carries these conditions 

 beyond the limits mentioned above, growth, and often life 

 itself, ceases. The kind of reaction produced by a change in 

 "the environment depends, of course, upon the nature of the 

 organism, upon the internal conditions, as much as upon the 

 outer world. Seeds of one form may develop rapidly when 



