2 (-6 Ethel McLennan : 



The seedling, the growth of which was to he determined, was fixed' 

 to the roof of a pressure chamber by means of plasticene. In the. 

 first observations the seedling was fixed to a plate of cork. A curious 

 error was noted- namely, the expansion and contraction of the cork 

 under varying pressures gave apparent growth movements to the 

 seedlings which did not actually take place. It is better to fix 

 seedling to plasticene by means of metal rods, test experiments 

 showed this was not affected by varying gas pressures. 



The radicle was so arranged, that it could he clearly seen from 

 the exterior through the glass ends of the chamber. 



The aii- was kept constantly moist by means of a lining of wet 

 cotton wool. A gauge was attached which registered the pressure to 

 which the seedling was subjected. The chamber was connected to a 

 high pressure pump, and so a pressure of any desired value could 

 lie produced in it. 



A horizontal microscope was used for the readings. It was levelled 

 carefully, and the tip of the root of the seedlings was fecussed on to 

 a scale in the eyepiece of the microscope. The divisions on the scale 

 = .064 of a mm. In order to be as accurate as possible, the eye 

 should he kept at the same level at each reading. To ensure this, a 

 rod of certain length was placed in the same position, and the 

 observer's chin rested on it at each reading. The initial position 

 having been read, the seedling is left for one hour, and then its 

 position is again read; this gives directly the amount of growth in 

 fractions of a mm. during that time. 



Before subjecting any seedling to pressure, the rate of growth 

 in air was first determined, for this varies according to the indi- 

 vidual, and having obtained its rate of growth in air a pressure of 

 known amount was developed in the chamber, and readings were 

 taken at intervals of an hour. 



The effect of the pressure does not manifest itself immediately I'll 

 the growth, at any rate such pressures as 1 have experimented with. 

 so apparently the direct mechanical effect of increasing the gaseous 

 pressure upon turgor is practically negligible as a factor which 

 influences growth, contrary to Pfeffer's suggestion. 



Generally in one day retardation became noticeable, the amount 

 of retardation being dependenl on the pressure; broadly speaking, 

 the higher the pressure the greater the retardation. 



It seems thai al such pressures as 1 have experimented with, this 

 retardation is not permanent, hut the plant accommodates itself 

 i" the pressure and the rate of growth is gradually raised. 



The temperature was noted throughout the experiments. 



