190 GROWTH 



at a higher temperature. The following figures, illustrating 

 the phenomenon in a culture of Botrytis, are relative to the 

 growth in air which is taken as ioo : — 



It is hardly necessary to remark that temperature and con- 

 centration of carbon dioxide are but two of the several factors 

 which influence the growth of these plants ; in addition, those 

 of nutrition, moisture and illumination are of importance.* 

 To return to the higher plants, the investigations of Leitch f 

 on the influence of temperature on the rate of growth of the 

 roots of Pisum sativum, show that the relationship can be 

 expressed as a uniform curve for the range of temperature 

 — 2° C. to 29 C. and resembles those of Kuijper for respiration. 

 Above 29 C. there is so much fluctuation that relationship 

 cannot be expressed in a single curve, wherefore for each higher 

 temperature a different curve must be made to express the 

 rate of growth in successive periods of time. This is owing to 

 the operation of one or more of those imperfectly known 

 factors termed by F. F. Blackman the time factor. For in- 

 stance, at 30 and 35 C. the rate of growth in the first ten 

 minutes is the highest attained, in the first half hour there is 

 a rapid fall followed by a recovery marked by a rise to a second 

 maximum, after which there is a gradual fall. At 40 C. the 

 decrease in growth rate is uniform and rapid, there being no 

 recovery. I As in the cress, observed by Talma, so in the pea ; 

 the coefficient for a rise in the temperature of ten degrees 

 shows a distinct falling off as the temperature rises, and, ac- 

 cording to the observations of Leitch, it is only between io° 

 and 22° that the coefficient value lies between 2 and 3, for 

 which reason the complete curve is not regarded as a van't 

 Hoff curve. The extremes of measurable growth was observed 

 at — 2° and 44-5° C, the highest rate being at 30-3° C. With 

 reference to these observations, Leitch distinguishes four 



* See, for example, Robinson : " Ann. Bot.," 1926, 40, 245. 

 f Leitch : id., 1916, 30, 25. 



X See also Sierp : " Ber. deut. bot. Ges.," 1918,35, 3; "Bot. Zentrlbl.," 

 1920, 40, 433. 



