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BOTANY 



is afforded by ordinary window-plants. The stems of such plants do 

 not grow erect as in the open air, but are inclined towards the window, 

 and the leaves are all turned towards the light as if seeking help. The 

 leaf-stalks and stems are accordingly positively heliotropic. In 

 contrast with these organs the leaf-blades take up a position at right 

 angles to the rays of light in order to receive as much light as possible. 

 They are diaheliotropic, or transversely heliotropic, in the strictest 

 sense (Fig. 201). If among the plants there should be one with aerial 

 roots, Chlorophytum for instance, an example of negative heliotropism 

 will be afforded, as the aerial roots will be found to grow away from 

 the window and turn towards the room. 



For more exact investigation of heliotropic movements it is 

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



Fig. 201. — Heliotropic curvature of a seedling of Galium Aparine, resulting from one-sided illumina- 

 tion ; in 1 the apex is in a line with the direction of the light, the leaves at right angles to it ; 

 in 2, with the illumination from the opposite direction, the same planthas quickly changed the 

 position of its apex, while the cotyledons are only beginning to assume their heliotropic 

 position. (Somewhat enlarged.) 



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 heliotropic 

 position ; every alteration in the direction of the rays produces a 

 change in the position of the heliotropic organs. 



The apical ends of 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 cryslallinus. The sporangiophores of this Fungus' are quickly- 

 produced on moist horse or cow dung. They are positively heliotropic, and turn 

 their dark 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 covering a small aperture through which the light has been admitted ; a 

 proof that the sporangiophores were all previously pointed exactly in that direction. 



Upon closer investigation of the manner in which the POSITIVE 



