110 THE AMERICAN BOTANIST 



form when surrounded by soil ; but as soon as it is freed from 

 that pressure, or rises above the ground it begins to straighten, 

 l)y the force of hyponasty. 



HEUOTROPISM AND GEOTROPISM 



The sun is one of the most vital agencies active on plant 

 Hfe. Stems and leaves behave very differently in relation to 

 the direction of light The stems grow toward the light 

 and the leaves arrange themselves so that the upper surfaces 

 are perpendicular to the sun's rays. Just the reverse of this 

 is the case with the roots; they grow away from the light. 

 This process of auto-turning is called heliotropism. Parts 

 which turn towards the light are described as positively heli- 

 otropic, while those which turn away are negatively heliotrop- 

 ic (apheliotropic) and those which turn across as transverse- 

 ly iieliotropic ( transheliotropic). There is a fourth case 

 which might be included under heliotropism. The leaves 

 of some plants when exposed to an intense or injurious 

 amount of light, protect their leaves by twisting or rolling 

 them. This is especially noticeable in dry wheather when the 

 plant seems to conserve its moisture by every way possible. 

 Such movements are sometimes called diurnal sleep or para- 

 heliotropism. 



When a plant is exposed to a lateral light the stems 

 move at first in a zig-zag line, which is undoubtedly due to 

 its circumnutation at the time, in a direction either opposite 

 to the source of light or transverse to it. As soon as the di- 

 rection of circumnutation nearly coincides with that of the 

 entering light, the plant bends in a straight line towards the 

 illuniinati(jn. The course becomes more and more rapid and 

 rectilinear in ratio to the intensity of the light and ellipses 

 tend to have a greater- ratio of diameters. 



