INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 267 



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

 trated 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 internodes 

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

 smaller leaf-blades prevail under dry conditions. The anatomical characters of 

 the two plants are likewise quite different. Plants that have been cultivated 



Fig. 118. — Bldens beckii. The Fig. 119. — Sagittaria sagittifolia. Lower, linear leaves 

 lower leaves have formed under formed under water; upper, arrow-shaped leaves formed 

 water and the upper ones in air. in air. 



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 Tropaolum majus 1 may serve 

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

 conditions, as shown in the table below, which also presents the results of 

 the experiment. 



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



