56 PLANT LIFE ON THE FARM. 



what manner roots, stems, and leaves are sensitive to the 

 effects of gravity, light, moisture, and actual contact or 

 irritation, and then to specify equally briefly what is the 

 general character of the results produced by these several 

 causes acting singly, or in combination. 



The Action of Gravity on Boots. Geotropism. The 



downward tendency of the main root is one of its most 

 marked characteristics, and this tendency to grow, or move 

 towards the centre of the earth under the influence of gravi- 

 tation, is known as " geotropism," the opposite tendency 

 being called " apogeotropism." Knight was the first to 

 show that the downward tendencies of the root were due 

 to gravitation, and this he did by causing seedlings to 

 grow on a wheel kept in motion. The effect of gravity 

 was here overcome by the movement of the wheel, and 

 the rootlets, instead of growing downwards, were now 

 directed away from the centre of the wheel. Darwin shows 

 (1. c., p. 540) that it is the tip of the root alone that is 

 involved in this downward tendency, the destruction of the 

 tip putting a stop to the movement. While the primary 

 root or radicle under favourable circumstances penetrates 

 the soil perpendicularly downwards, the secondary ones 

 bend obliquely, not perpendicularly, downwards, the ter- 

 tiary ramifications and their subdivisions being so little 

 affected by geotropism that they grow out freely in all 

 directions. From this manner of growth in the main root 

 and its branches respectively, it is evident how the whole 

 mass of soil within their reach becomes, under favourable 

 conditions, a happy hunting ground for the roots. More- 

 over, it has been shown that where the primary radicle, 

 the origin of the "tap'* root, has been destroyed as 



