56 THE QUANTITATIVE METHOD IN BIOLOGY 



exact knowledge by means of the quantitative method, ^ the 

 properties being measured after n, 2n, yt . . . hours, the figures 

 being plotted down in the form of curves. (See, on the use of a 

 leading property, § 49.) 



Let us now consider the corolla of the same species when 

 grown at a temperature of 30° C. The properties length and 

 breadth behave on the whole in the same way as in the preced- 

 ing case. The primordium white is original. Since the reaction 

 which produces the primordium red does not take place, white 

 is arrested (its value is invariable and may be represented by 

 O = no colouring substance) and persistent (instead of transitory) . 



In the preceding example only three primordia (colour, 

 length, breadth) have been considered. It is, of course, possible 

 to devote attention to a larger number of primordia at the same 

 time ; for instance, the number of epidermal ceUs and their 

 dimensions, the number of stomata and the dimensions of 

 their cells etc. 



SECOND EXAMPLE (ACCRESCENT PRIMORDIUM): 

 In a leaf of a tree we may distinguish, among other primordia 

 (limiting ourselves to the limb) : (i) the length (measured along 

 the mid-nerve) ; (2) the breadth (at the place of the greatest 

 breadth) ; (3) the number of palissade cells (counted in the 

 transverse direction at the place of the greatest breadth) ; (4) 

 the breadth of the palissade cells (calculated by dividing the 

 breadth by the number of cells). 



The primordium number of cells increases from the beginning 

 till the maximum is reached : it is original, arrested, persistent. 

 During this first period of the development of the leaf (period 

 of ceU-division) the breadth of the cells may be looked upon as 

 being constant. At (or about) the end of this period the cells 

 begin to grow, and this process goes on till they have reached 

 their full size. The primordium breadth of the cells is original, 

 arrested and persistent. Since the increase of this primordium 

 is continued after the primordium number has been arrested, it 

 might be called ACCRESCENT in comparison to the primor- 

 dium number. The primordia length and breadth of the limb 

 are both original, arrested and persistent. 



SECOND EXAMPLE {continued). SENSITIVE PERIOD, 

 CORRELATION. The period of development of the leaves of 

 any of our indigenous trees (for instance, Fagus silvatica) is 

 divided by the winter rest into two parts : (i) the bud period 

 (intragemmal period), which corresponds to the summer (July- 

 September) of the first year ; (2) the spring period (vernal 

 period), which begins when the bud expands, and extends (in 



1 Length and breadth may be easily measured. The measurement of 

 colours is possible in several ways (scale of colours, chemical analysis, 

 spectroscope). 



