162 ROYAL SOCIETY OF CANADA 



material, i.e., quartz. The agar medium is transparent, so that the 

 roots are at all times visible and growth and nodule formation may be 

 observed from the start. The quartz is opaque and the roots cannot 

 be seen except where they press again the sides and bottom of the 

 flask. The aga,r medium is a firm jelly with a glossy surface and 

 the growth of Ps. radicicola can be measured or observed on the surface 

 or as it accompanies the roots. Contaminations are easily detected, 

 because of the formation of visible colonies. The quartz medium is 

 a mixture that may be regarded as a liquid medium, yet the growth 

 of Ps. radicicola or any possible contamination cannot be directly 

 observed nor detected. Both afford good support for root and stem; 

 both are favourable for the growth of Leguminosae and for the form- 

 ation of nodules. In the quartz medium, root hairs form in those 

 portions which are not sa,turated, and in the agar medium root hairs 

 form on roots which creep over the surface of the agar but are but 

 little developed on roots which penetrate the agar. 



Growing the plant within a glass flask aflords several advantages 

 and offers few technical difficulties. It makes possible the most rigid 

 pure culture methods; it requires no attention l^eyond the initial pre- 

 paration; that is, the medium does not require to be restored nor 

 renewed, even during a period of growth of eight months. 



In these flask cultures the plant is nourished as follows: — Sun- 

 shine, the energy for growth, comes through glass walls of the flask; 

 the essential ash elements are supplied in the form of a filtrate from wood 

 ashes, the oxygen, carbon dioxide and free nitrogen, pass in and out 

 through the cotton plug in the mouth of the flask ; water is abundantly 

 supplied from the agar jelly, which consists of about 98.5 per cent 

 water. The plant, being within the flask, the water of evaporation 

 and transpiration is nearly all condensed on the glass and returned to 

 be used) over and over again. In other experiments where the stems 

 and leaves were freely exposed to the outer air, the water of transpir- 

 ation was lost and, consequently, the medium was soon exhausted. 

 Before entering the host plant, the culture of Ps. radicicola is nourished 

 in like manner, but gets its energy of growth from the oxidation of 

 the malt sugar supplied in the medium. 



