CHAPTER VII. 



MOVEMENT IN PLANTS. 

 I. PHOTOTROPISM (HELIOTROPISM). 



282. Positive Fhototropism. A familiar example of 

 tropistic movement is the turning of shoots towards the 

 light, as shown by plants growing near a window. Ordi- 

 nary erect (orthotropic) shoots grow towards the source 

 of unilateral illumination, the stem tending to place its 

 axis parallel with the direction of the light (positive photo- 

 tropism) , while the leaves place their surfaces at right angles 

 to it (diaphototropism) . 



(a) Grow seedlings of Bean, Wheat, Sunflower, etc., in 

 darkness : the shoots are erect. Now place them in front 

 of a window, or in some other position where the light 

 falls on them mainly from one side, and note the changed 

 direction of growth of the shoots. When marked curva- 

 ture has taken place, turn the seedlings round again, 

 through 180, and note the result. 



(6) Place in each compartment of a phototropic or dark 

 chamber a pot plant, or some seedlings, or a cut shoot in 

 a bottle of water. Make one compartment light-tight, 

 but allow light to enter the other through a slit in the side. 

 Try various plants. A very rapid reaction is given by 

 seedlings of Tare (Vetch) and of various Grasses (especi- 

 ally Millet, Italian Millet, and Canary Grass) that have 

 been germinated in darkness ; Phaseolus shoots are less 

 sensitive, while cut young shoots of Elder, etc., react rather 

 slowly. 



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