INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 237 



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 waUs 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 hkewise quite dififerent. Plants that have been cultivated 



Fig. 118. — Bidensbeckii. The Fig! 119. — Sagittaria sagittaifoUa. 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 dryjsoil and dry air have a thick cuticle, well-developed coUenchyma, and 

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

 and poorly developed woody tissue, and coUenchyma and bast fibers are often 

 not formed at all. An experiment with Tropaolum majus^ may serve as an 

 example here. The plants were cultivated under four different sets of condi- 

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

 experiment. 



•Kohl, 1886. [See note 3. P- 123-1 



