PHYSIOLOGY 



275 



Upon them the light has a most injurious, even fatal, effect, as may 

 be easily observed in the case of Fungi and Bacteria. The hygienic 

 importance of daylight in dwelling-places is due to the destructive 

 action of light upon such forms of plant life. That some plants seek 

 the light, while others avoid it, is not surprising in view of the 

 adaptability which organisms usually exhibit in respect to the in- 

 fluences with which they come 

 in contact in the natural course 

 of their development. 



A good opportunity for the 

 observation of heliotropic pheno- 

 mena is afforded by ordinary 

 window-plants. The stems of 

 such plants do not grow erect 

 as in the open air, but are in- 

 clined towards the window, and 

 the leaves are all turned towards 

 the light as if seeking help. 

 The leaf-stalks and stems are 

 accordingly POSITIVELY HELIO- 

 TROPIC. 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 TRANS- 

 VERSELY HELIOTROPIC, in the 

 strictest sense (Fig. 223). If 

 among the plants there should 

 be one with aerial roots, Chloro- 

 phytum 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 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 heliotropic position; every 

 alteration in the direction of the rays produces a change in the posi- 

 tion 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. 



FIO. 2-23. A sci-aimx of the white Mustard m a 



wat " r culture w * i< ? 1 has flrst bee illumi " :ltt " 1 

 from all sides and then from one side only. 1'hc 



stem is turned towards the light, the root away 

 from 'k> wlli ''' tin- leaf-blades are expandi-d ;it 



