170 



OUTLINES OF PLANT LIFE. 



therefore, to consider the effect of each stimulus upon the 

 two common forms of plant organs — namely, the radial (such 

 as stems and roots) and the flattened (such as leaves). 



243. (a) Heliotropism. — Heliotropism is the state of a 

 plant or organ when it is irritable to the direction of light 

 rays. Light thus plays an important part in determining the 

 position of organs. As a rule radial organs are either posi- 

 tively heliotropic, as the stems and leaf-stalks, or negatively 

 heliotropic, as the roots. In ordinary light leaves are all 

 transversely heliotropic, assuming a position at right angles 

 to the direction in which the light, comes. This is the most 

 favorable position possible for the manufacture of food by 

 the green parts (fig. 124). Intense light, however, may- 



u n 



Fig. 124.- Diagrams representing the transverse heliotropism of leaves of the garden 

 nasturtium (Tropmoluni). Potted plants were subjected successively to light strik- 

 ing them in the direction shown by arrows. The petioles curved so as to place the 

 blades at right angles to the incident light.— After Vochting. 



bring about a different reaction, so that the leaves set them- 

 selves edgewise to the light. A fixed light position is usually 

 reached by leaves by the time they become mature, and this 

 is generally at right angles to the source of greatest light. 

 Branches of trees show the leaves so arranged as to size and 

 position that they shade each other as little as possible, form- 

 ing the so-called leaf mosaics (figs. 125, 126). The leaves of 

 window plants also exhibit these movements very strikingly, 



