THE MOVEMENTS OF ELAXTS. 1 95 



Light thus jjlays an important part in determining the position 

 of organs. As a rule radial organs are cither positively heli- 

 otropic, as the stems and leaf-stalks, or negati\el)' heliotropic, 

 as the roots. Dorsiventral organs, such as leaves, are all 

 transversely heliotropic, assuming a position at right angles 

 to the incident rays, which is the most favorable position 

 possible for the manufacture of food by the green parts (fig. 

 190). Intense light, ho\ve\-er, may bring about a different 

 reaction, so that the leaves set themselves edgewise to the 



Fig. iQr. — Leaf mosaic formed by a horizontal shoot of Norway maple. The lengthen- 

 ing of the petioles of individual leaves to avoid shading of the blade is marked. 

 About one-third natural size. — After Kemer. 



direction of the rays. 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. 191 to 193I. The leaves 

 of window plants also exhibit these movements very strikingly, 



