38 PEOPAGA.TION- OF PLAITTS. 



ficient uniformity in the laws governing the growth of 

 vegetation to enable us to discover what we may term 

 general principles, yet it is impossible for us to determine 

 with certainty the exact limits of variation. 



In a general way, it may be said that roots growing in 

 the ground have a tendency to go downward, or toward 

 the center of the earth. Prof. Gray says that the plant- 

 let possesses a kind of polarity, and is composed of coun- 

 terpart, systems — namely, a descending axis, or root, and 

 an ascending axis, or stem. Prof. Balfour, in referring 

 to this subject, says : "Physiologists have not been able 

 to detect any law to which they can refer the phenomena, 

 although certain agencies are obviously concerned in the 

 effect. Some have said that the root is especially influ- 

 enced by the attraction of the earth, while the stem is 

 influenced by light." But experiments have shown that 

 the downward course of the root is not always due to the 

 attraction of gravitation, or to moisture in the soil ; nei- 

 ther is the ascent of the stem due to the action of light, 

 although all these, no doubt, have an influence upon the 

 plantlet in its early stages of growth. Thomas Andrew 

 Knight, placed mustard seeds and French beans on the 

 circumference of two wheels which were put in rapid 

 motion, the one horizontal and the other in a vertical 

 manner, and he found that in the former the roots took 

 a direction intermediate between that impressed by gravi- 

 tation and by the centrifugal force — namely, downward 

 and outward, while the stems were inclined upward and 

 inward. In the latter the force of gravitation was neutral- 

 ized by the constant change. of position ; the centrifugal 

 force acted alone, by which the roots were directed out- 

 ward at the same time that the stems grew inward. But 

 these variations may have all been due to the liquid con- 

 dition of the parts of the young plants. Some plants 

 grow indifferently in all directions from the very incep- 

 tion of germination, and it is well known that the roots 



