SECT. II 



PHYSIOLOGY 



303 



In correspondence with the changes in the position of tlie chloroplasts, the 

 colouiiug of green organs naturally becomes modified. In direct sunshine they 

 appear lighter, in diffused light a darker green. The attention of Sachs was first 

 called to the phenomena of the movements of the chloroplasts by the accidental 

 observation that the shadow of a thermometer was represented in dark green on a 

 leaf otherwise directly illuminated by the sun. 



Phototpopism (Heliotropism) {^^') 



A good opportunity for the observation of heliotropic phenomena 

 is aftbrded 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. The 

 leaf- stalks and stems are accord- 

 ingly ORTHOTROPIC and POSI- 

 TIVELY PHOTOTROPIC. In con- 

 trast 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 DIA- 



PHOTOTROPIC, or TRANSVERSELY 



HELIOTROPIC, in the strictest 

 sense (Fig. 228). If among 

 the plants there should be one 

 ■with aerial roots, Chlorophylum 

 for instance, an example of 



NEGATIVE PHOTOTROPISM will 



be afforded, as the aerial roots 

 Avill be found to grow away 

 from the window and turn 

 towards the room. In Fig. 

 228 the phototropic curvatures 

 which take place in a water 

 culture of a seedling of the 

 White Mustard are represented. 



K 1 



Fv 



For more exact investigation 



:. 228. — A seedling of the .Wliitc Mustard in a 

 water culture wliicli lias lirst 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. 



of phototropic movements it is 

 necessaiy 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 plioto- 

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



