160 THE AMERICAN BOTANIST 



Of all the stimuli made use of by plants for advantageous guid- 

 mg of their parts, gravitation is by far the most important. 

 Plants have in themselves an hereditary tendency to put forth 

 their parts in a symmetrical manner, l)ut tlicy depend upon 

 geotropism to guide those parts to the suitable positions and 

 thus realize the ultimate shape of the plant. This is applicable 

 not only to the main structures, but to most of the minor parts 

 being the case especially with roots. The fact that geotropism 

 is thus ever tending to hold the plant in a certain upright sym- 

 metrical form explains why any one-sided turning in response 

 to other stimuli is of a limited amount, and why the plant al- 

 wavs tends to recover its former upright and symmetrical po- 

 sition in case it is disturbed. 



Apogeotropism is the reverse of geotropism. Different 

 species and different parts of the same species are acted on by 

 apogeotropism in very different degrees. Young seedlings, 

 most of which circumnutate quickly and largely bend upwards 

 and become vertical in much less time than do other plants, but 

 whether this be due to their greater sensitiveness to geotro- 

 pism, or merely to their greater flexibility is not definitely 

 known. 



A part or organ w^hich is extremely sensitive to geotro- 

 pism ceases to be so as it grows old and it is remarkable as 

 showdng the independence of this sensitiveness and of the cir- 

 cumnutating movement, that the latter sometimes continue for 

 a time after all power of bending by gravity has been lost. 

 Also geotropism acts when \'ery nearly balanced by an oppo- 

 sing force. When the stem of any plant bends during the 

 day towards a lateral light, the movement is opposed to apo- 

 geotropism, but as the light gradually w anes in the evening the 

 latter power slowly becomes more powerful and draws the 

 stem back into a vertical position. A stem or other organ 



