30 



plants. The air-spaces of some roots are said to be re- 

 latively larger toward, and in, the growing tip than in 

 the older, less active portions. 2 Amphibious plants are 

 reported to develop more and larger aerating systems 

 when grown in water than on land. 3 Finally, a number of 

 swamp plants are known to provide themselves with special 

 organs for internal root aeration. The best known of 



1. Goebel,- Pflanzenbiol. Schilderungen, vol. 

 2, p. 249 (1893); Warming,- Oecology, pp. 44, 98 (1909); 

 Haberlandt,- Physiological Plant Anatomy, p. 441 (1914), 

 There are, however, many exceptions; cf. Goebel,- loo , cit. 



2* Hunter,- Ann. Bot. 29: 627-634 (1914). 



3. Stahl,- Einf. sonnigen und schattigen 

 Standorts, p. 17 (1883); Schenck,- Biol, der Wassergewachse, 

 p. (1886), Jahrb. wiss. Bot. 20: 526 (1889); 

 Goebel,- Pflanzenbiol. Schilderungen, vol. 2, pp. 255*256 

 (1893); Wieler,- Jahrb. wiss. Bot. 32: 519 (1898); Gluck,- 

 Wasser- und Sumpfgewachse, vol. 1, pp. 268-70, (1905). 

 Gluck ( loc . cit . ) notes that some of the modification pro- 

 duced by growth in water may be a response to change of 

 light intensity rather than to aeration. 



