262 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



pumped out of the vessels in which the cultures are grown, or the oxygen may 

 be absorbed from the air of the culture vessels by means of a solution of pyro- 

 gallol and potassium hydroxide. The test-tube containing the culture is placed 

 within another larger test-tube, which is partially filled with alkaline pyrogallol 

 (P, Fig. 107). The larger tube is tightly closed with a rubber stopper, the 

 oxygen is absorbed by the alkaline pyrogallol, and the bacteria of the inner tube 

 are thus exposed to an atmosphere without oxygen. 



The form of the plant may also be controlled by the oxygen content of the 

 surroundings. Thus Mucor, a very common mould, develops a much-branched 



n. 



m 



n 



Fig. 107. — Culture 

 of anaerobes. 



Fig. 108. — Mucor mucedo, showing mycelium and sporangiophores. 



mycelium in the presence of oxygen, and produces vertical sporangiophores that 

 grow up from the mycelium, sometimes attaining a length as great as 10 cm. 

 (Fig. 108). If, however, the mycelium is grown in the bottom of a flask filled 

 with beer- wort, where the supply of oxygen is inadequate for the usual growth, 

 then alcoholic fermentation begins and the mycelium divides into single cells, 

 which become separated and resemble those of ordinary yeasts. Thus arises 

 the so-called mucor yeast (Fig. 109). This example represents an extreme case 

 of the influence of the medium upon the form of organisms. 



§3. Influence of Other Gases on Growth and Configuration. — Plants 

 grow normally only when the air about them has its usual composition. The 

 carbon dioxide content of the atmosphere is about 0.03 to 0.04 per cent. The 

 investigations of Brown and Escombe 1 and those of Chapin 2 showed, in a 

 quite unexpected way, that an increased carbon dioxide content of the atmos- 



1 Brown, Horace T., and Escombe, F., The influence of varying amounts of carbon dioxide in the air on 

 the photosynthetic process of leaves and on the mode of growth of plants. Proc. Roy. Soc. London 70: 

 397-4I3- 1902. 



2 Chapin, Paul, Einfluss der Kohlensaure auf das Wachsthum. Flora 91 : 348-379. 1902. 



