29 



such as lemperjiture, rainfall, season, sunliq^ht, soil conditions and 

 available ))lant food. An attempt was made to isolate some of 

 llu'se factors by j^ro\\'ini4- a number of series of peas in water cul- 

 lures throuj^iiout a period of sixteen months, results bein^ thus 

 obtained for all seasons of the year. Measurements of maximum 

 and minimum temperatures and number of hours of bright sun- 

 shine were recorded throughout, and provided a basis for statistical 

 correlations. Parallel series were usually grown, in one of which 

 ihe nutrient solutions \\ ere changed weekly so that an abundant 

 food supply was assured, whereas in the other the solution was not 

 renewed, and the food supply was severely restricted. 



It was found that growth may be divided conveniently into 

 twf) well-marked periods. 



{(i) 1st period, from the seedling stage till the time that the 

 l)lant 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." 



{!)) 2nd period, succeeding the former, during which the plant 

 is obviously making growth, and whi(h continues till the latter 

 ceases and dessication sets in. 



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 

 temperature, up to a certain limit. 



The possible amount of growth, as measured by the dry matter 

 |)roduced, depends entirely u]3on the bright sunshine and tempera- 

 ture when th(; 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. Heyond a certain limit, however, the 

 beneficial factors of heat and l>right sunshine become harmful and 

 result in the premature death of the plant. 



During the first period the rate of growth, as shown by the 

 elliciency index, was associated with relatively warm days and 

 nights, bright sunshine having little significant effect ; the lig'ht, 

 however, was good throughout for the season of the year. During 

 the second period the rate was associated strongly witli sunshine 

 and warm davs, but not significantlv with the night temperatures, 

 which did not' fall below 32- 1-. 



During- the greater part of the vear the maximum rate of 

 growth is reached early in life, but in winter, when temj)eratures 

 are low and there is little bright sunshine, the maximum rate is 

 not attained till much later. 



Plants with a restricted food supply make less total growth 

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

 dry matter j)roduced does not seem to be gradual but is marked by 

 definite periods, of which the incidence varies at different seasons. 



During the period of actual growth, the shoot increases in 

 weight far more rapidly than the root. Increase in shoot growth 

 is closely associated with rise in temperature and root growth is 

 adversely affected by low mean maximum temperatures. Rise in 

 maximum temperature has much less beneficial action upon the 

 roots than upon the shoots. 



In early stages of growth, the amount of nitrate absorbed by 

 the plant is relatively large in comparison with the dry matter pro- 

 duced, but later on more dry matter is formed in proportion to the 



