254 GROWTH 



ments in which the organ stimulated curves away from the light, 

 like roots. 



The utility of these movements is clear : by such movements 

 stems are enabled to reach the light and so place the leaves 

 which they bear in the most favourable position for carrying on 

 their function of ' carbon-fixation,' and roots are aided in finding 

 their way and penetrating into the dark crevices of the soil. 



The leaves of an onion and the flat sword-like leaves of 

 certain monocotyledons appear to be heliotropic like stems, but 

 the majority of the ordinary green leaves of plants behave 

 differently from either roots or stems. They usually turn or 

 twist on their petioles so as to place the upper surfaces of 

 their blades at right angles to the direction in which the light 

 falls upon them ; plant members taking up such a position in 

 reference to the incident light are described as diaheliotropic. 



A few stems, such as those of the ivy, appear to be dia- 

 heliotropic. Instead of bending away from a wall they grow 

 close up to it, and need no special training to keep them there. 

 The ordinary heliotropic stems of fruit-trees, however, growing 

 in a similar situation curve away from the wall, and if this is 

 to be prevented the growing tips must be secured until they 

 have become mature and firm. 



Experiments have proved that only the blue and violet rays 

 of light are effective in inducing heliotropic movements : no 

 response is made to red and yellow rays. 



Ex. 166. — Sow some mustard seeds in two small three-inch flower-pots, 

 and when the plants are about an inch high place one pot of the seedlings 

 in a box in total darkness, the other pot cover with a box blackened in the 

 inside with lamp black, and having a hole bored in one side about on a 

 level with the top of the seedlings. 



Allow the seedlings to grow, and in a day or two compare the direction 

 of growth of their stems in the two pots. 



Ex. 157. — Germinate a few mustard seeds in damp sawdust, and when 

 their primary roots are about an inch or an inch and a half long take one 

 or two of the seedlings and push their roots through holes in a strip of 



