Winifred E. Brenciiley 243 



Summary. 



1. Growth may be divided conveniently into two well-marked periods. 

 (a) ist period, from the seedling stage till the time that the plant 



regains its initial weight after the loss by respiration, i.e. the time during 

 which a casual observer would say the plant '' makes no growth." 



(6) '2nd period, succeeding the former, during which the plant is 

 obviously making growth, and which continues till the latter ceases and 

 desiccation sets in. 



2. The length of the first period varies inversely with the mean 

 maximum temperature, as the rate at which assimilation is able to 

 make good the loss by respiration increases directly with rise of tem- 

 perature, up to a certain limit. 



3. The possible amount of growth as measured by the dry matter 

 produced depends directly upon the bright sunshine and temperature 

 when the food supply is adequate, but when the latter is limited the 

 total growth is much less owing to the lack of material for building up 

 the tissues. Beyond a certain limit, however, the beneficial factors of 

 heat and bright sunshine become harmful and result in the premature 

 death of the plant. 



4. During the first period the rate of growth as shown by the 

 efficiency index was associated with relatively warm days and nights, 

 bright sunshine having little significant effect; the light, however, was 

 good throughout for the season of the year. During the second period 

 the rate was associated strongly with sunshine and warm days, but not 

 significantly with the night temperatures, which did not fall below 32° F. 



5. During the greater part of the year the maximum rate of growth 

 (highest efficiency index) is reached early in life, very soon after the 

 second period begins. Under favourable environmental conditions a 

 high rate of increase is then maintained for several weeks, but in less 

 favourable circumstances the efficiency index rapidly falls. In winter, 

 when temperatures rule low and there is little bright sunshine, the maxi- 

 mum rate of growth is not reached till several weeks after the beginning 

 of the second period, and even then the efficiency index is not very great. 



6. Plants with a restricted food supply make less total growth than 

 those with abundant food. The falling off in the amount of dry matter 

 produced does not seem to be gradual but is marked by definite periods 

 of which the incidence varies at different seasons. 



7. Broadly speaking the response of plants to the (environmental 

 conditions is similar whether the food supply is abundant or restricted, 



Ann. Biol, vi 17 



