DIV. II 



PHYSIOLOGY 



349 



ment, and the primary root of the Mistletoe, turn from the light towards, and are 

 pressed firmly against, their darker supports. 



For more exact investigation of phototropic movements it is neces- 

 sary to be able to control more accurately the source and direction 

 of the light. This can be best accomplished by placing the plants in 

 a room or box, lighted from only one side by means of a narrow 

 opening or by an artificial light. 

 It then becomes apparent that 

 the direction of the incident rays 

 of light determines the photo- 

 tropic position ; every alteration 

 in the direction of the rays 

 produces a change in the position 

 of the phototropic organs. The 

 apical ends of many positively 

 heliotropic organs will be found 

 to take up the same direction 

 as that of the rays of light. 



The exactness with which this is 

 done is illustrated by an experiment 

 made with Pilobolus crystallines (Fig. 

 284). The sporangiophores of this 

 fungus are quickly produced on moist 

 horse or cow dung. They are posi- 

 tively phototropic, and turn their 

 black sporangia towards the source 

 of light. When ripe these sporangia 

 are shot away from the plant, and 

 will be found thickly clustered about 

 the centre of the glass over a small 

 aperture through which alone the 

 light has been admitted ; a proof Fi<;. 283. A seedling of the White Mustard in a 



that the sporangiophores were all 

 previously pointed exactly in that 

 direction. 



water culture which has first been illuminated 

 from all sides and then from one side only. The 

 stem is turned towards the light, the root away 

 from it, while the leaf blades are expanded at 

 right angles to the incident light. KK, Sheet of 

 cork to which the seedling is attached. (After 

 XOLL.) 



The positive phototropic 

 curvatures are brought about 



by THE SIDE TURNED TOWARDS 



THE LIGHT GROWING MORE SLOWLY, AND THAT AWAY FROM THE LIGHT 



MORE ACTIVELY, THAN UNDER ILLUMINATION FROM ALL SIDES. The 



converse distribution of growth is found in negative phototropism. 

 As a rule CURVATURES ONLY TAKE PLACE IN THE REGION WHICH is 



STILL IN A GROWING CONDITION, THE SHARPEST CURVATURE BEING AT 

 THE REGION OF MOST ACTIVE GROWTH. 



The course of phototropic. curvature shows a complete correspondence with 

 geotropic curvature (p. 341). A. ENGLER has recently demonstrated phototropic 



