174 



GROWTH 



carbon assimilating area per unit of dry weight (Fig. 8). The 

 correspondence, however, is not precise ; instead of showing 

 a definite type of variation, as does the relative growth rate, 

 the leaf area per unit dry weight curve fluctuates about a 

 mean value. These fluctuations are due to the conditioning 

 factors of growth of which factors temperature is amongst the 

 more significant. The importance of leaf area in the economy 

 of the higher plant is so obvious that no elaboration of the 

 statement is required. The growth in area of the leaves of 

 the cucumber has been closely studied by Gregory * and may 

 be alluded to at this stage in that it introduces some aspects 

 of growth which properly belong to a general consideration. 

 But before this is done, the " grand period of growth " must 

 be recalled to memory. As is well known, Sachs used this 

 expression to designate the period through which the plant, or 

 plant member, exhibits its sequence of growth rates. Thus 

 the growth rate of the first internode of Phaseolus multiflorus 

 is at first slow, then quickens to a maximum, after which a 

 decrease in the rate to zero obtains, which point is coincident 

 with the attainment of maturity. The sequence is illustrated 

 in Fig. 9 which is based on Sachs's measurements. 



The grand period of growth does not include an analysis 

 of the fluctuations in the rate during the periods between the 

 measurements ; thus the growth rate between the third and 

 fourth day is the summation of the growth during that period. 

 Priestley and Evershed f in their study on the root growth of 

 Tradescantia and tomato, based on the increase in dry and wet 

 weights of the roots produced on cuttings, find the curves 

 obtained are a sequence of these grand period curves which 

 are a series of S curves, a type which is very common in graphs 

 illustrating growth phenomena (see Figs, y and io). The time 

 of transition from one S curve to the next coincides with the 

 time of appearance of a crop of roots of a subordinate branch 

 order. 



Gregory found that the growth in length, in breadth, and 

 in area of the leaves of Cucumis sativus show a grand period 



* Gregory : " Ann. Bot.," 1921, 35, 93. 



t Priestley and Evershed : id., 1922, 36, 225. 



