GRO WTH. 



75 



EXPERIMENT 96. 



COMPARISON OF THE GROWTH OF CUTTINGS IN LIGHT AND DARKNESS. 



Fasten a Willow twig in an upright position in a covered glass 



cylinder containing some water, and set in the sunlight. It develops 



roots below and shoots above. Another twig treated in the same 



Fig. 68. manner but placed in the dark acts similarly. (Fig. 



68.) 



EXPERIMENT 97. 



INFLUENCE OF LIGHT ON THE FORMATION OF ROOTS AND SHOOTS. 



Set up the experiment as above, but place the cylin- 



FlG. 69. 



der in a pasteboard box which 

 admits light at the side through 

 a long slit. Roots develop on the 

 side of the shoot away from the 

 light, and shoots on the illumi- 

 nated side. 



EXPERIMENT 98. 



DEVELOPMENT OF ROOTS AND SHOOTS 

 IN A REVERSED POSITION. 



Suspend a Willow twig in a 

 reversed position in a glass cylin- 

 der furnished with water, in the 

 light. A contest arises between 

 the specific tendency of the twig 

 to form shoots on the original 



,. „,.„ , . . 1 •*• upper end, and roots on the lower 



68. Willow twig in normal position. ^'^ ' 



(Hansen.) a, shoots ; b, roots. end, and the influence of light 



69. Willow twig in reversed position, and gravity which directly op- 

 (Hansen.) a, original upper end ; pose it. At first the habit of 

 b, original lower end. ^^^ pj^^^^ prevails, and roots are 



formed on the upper, shoots on the lower end. Then the influence 

 of the physical forces is manifested by the development of roots on 

 the lower and shoots on the upper end of the twig. (Fig. 69.) 



57. Influence of Temperature on Growth. — For every plant 

 there are five important temperature divisions : ist, destructive 

 cold, a low^ temperature producing death by the disorganiza- 

 tion of the protoplasm ; 2d, specific zero, which arrests the 

 activity of the protoplasm, but does not necessarily result in 



