INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



269 



ducted by placing the plant under a bell-jar, with a vessel of calcium chloride or 

 concentrated sulphuric acid to reduce the vapor pressure of water. To obtain 

 moist conditions, a sponge saturated with water may be introduced into the bell- 

 jar and the walls of the latter may be moistened. The plant develops long inter- 

 nodes and broad leaf-blades in a moist atmosphere, but short internodes and 

 much smaller leaf-blades prevail under dry conditions. The anatomical charac- 

 ters of the two plants are likewise quite different. Plants that have been culti- 



Fig. 118. — Bidens beckii. The 

 lower leaves have formed under 

 water and the upper ones in air. 



Fig. 119. — Sagitlaria sagittifolia. Lower, linear leaves 

 formed under water; upper, arrow-shaped leaves formed 

 in air. 



vated with dry soil and dry air have a thick cuticle, well-developed collenchyma, 

 and both bast and wood fibers. Plants grown under moist conditions have thin 

 cuticle and poorly developed woody tissue, and collenchyma and bast fibers are 

 often not formed at all. An experiment with TropcBolum majus 1 may serve 

 as an example here. The plants were cultivated under four different sets of 

 conditions, as shown in the table (page 270) which also presents the results of 

 the experiment. 



!Kohl, 1886. [See note 3, p. 135.] 



