WiNiFRKi) E. Brenciiley 237 



General Observattons on Growth. 

 (1) Comparison of root and shool yioirl/i. 



Inspection of the growth curves shows that at all stages of growth 

 and at all seasons the dry weight of the root is less than that of the shoot. 

 As the plants get older the shoot becomes rapidly heavier under favour- 

 able conditions, but the increase in root weight is by no means parallel 

 to that of the shoot as it is very much slower, with the result that when 

 very heavy growth is ultimately made by any plant the discrepancy in 

 the dry weight of the root and shoot is very marked. This change in the 

 relations between the two parts of the plant is well shown by the ratio 

 between the dry weights of shoot and root for the weekly periods for 

 which figures are available (Figs, 12 and 13). 



In the very young seedlings, at the stage at which they are put into 

 the food solutions, only part of the material stored in the seed has been 

 utilised and the roots are as yet very small. Since the seed is included 

 with the shoot the shoot/root ratio at this time is very high, ranging from 

 about 14-32 according to circumstances^. From this time more normal 

 relations establish themselves, for the seed store becomes rapidly depleted 

 and the ratio is that between the actual shoot (stem and leaves) and the 

 root. The shoot/root ratio continues to fall at a decreasing rate for a vary- 

 ing period, the lowest ratio corresponding fairly closely with the end of 

 the first period of growth, though it may be reached a week or two earlier 

 or later. From this time onwards, when there is no deficiency of food 

 (Fig. 12), the shoot increases more rapidly than the root in weight, and 

 the shoot/root ratio goes steadily up. The more rapid the growth, the 

 more marked this rise, and in the summer months under optimum 

 conditions of temperature and light the low proportion of root to shoot 

 is most striking. When the food supply is limited (Fig. 13) and less 

 growth is made the figures do not reach such a high level, but exactly 

 the same course of events is noticed, as a rise in the shoot/root ratio 

 occurs from about the end of the first growth period and the largest 

 ratios are obtained in the summer months. 



An explanation of this change in the proportion of shoot and root 

 may be found in the different mechanical construction of the two parts. 

 The two main functions of the root are the absorption of water con- 

 taining dissolved food substances and the fixing of the plant in the sub- 

 stratum. For the efficient performance of the first function a large area 



^ For economy of space this initial high ratio is omitted from Figs. 12 and 13, the first 

 ratio on the curve being that obtained after one week's growth in nutrient solution. 



