GROWTH CURVES 



565 



"growth curves" are usually plotted in terms of rate of growth or total 

 growth increment against time. The rate of growth or growth increment 

 may be expressed in terms of any of the commonly accepted indices of growth — 

 elongation, enlargement, volume increase, dry weight increase, or fresh weight 

 increase. Since these indices are principally or entirely measures of the cell 

 division and cell enlargement phases of growth, curves plotted in terms of 

 such units represent only the quantitative phases of growth. 



IVIeasurements of the rate of growth (increase in length per day) and 

 total growth increment (cumulative length at time of measurement) for the 

 hj'pocotyls of the muskmelon {Cucurnis iiuio) are shown in Table 56. 



TABLE 56 RATE OF GROWTH AND GROWTH INCREMENT OF MUSKMELON COTYLEDONS AT 20° C. 



(data of EDWARDS, PEARL, AND GOULD, I934) 



If these data for the rate of elongation be plotted against time a curve 

 such as that shown in Fig. I22, A will result. Similar curves will result 

 if the rate of increase in size of any plant cell or organ be plotted against 

 time. The general trend of all such curves indicates that elongation is at 

 first slow, then steadily increases until a maximum rate is attained, after 

 which a slow but steady diminution in rate sets in, until eventually increase 

 in the size of the cells ceases entirely. Similar curves will usually result if 

 the rate of increase in fresh weight or rate of increase in dry weight is taken 

 as a quantitative index of growth instead of the rate of enlargement. Every 

 plant cell and hence every coordinated group of cells undergoes such a cyclic 

 change in the rate of enlargement during the growth period. The time period 



