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Relatione between Growth and Envu<Hnn(itt 



increase but the restricted ones fall oft' in the manner indicated above. 

 In the summer with optimum conditions the extra food supply is 

 beneficial from the first as the plants have a very high efficiency index 

 and are able to utilise the nutrients to much better advantage, but though 

 the restricted plants cannot keep pace with the others they rise in weight 

 quite steadily for several weeks before depression sets in. It is only in 

 series grown inclusively from March to June that the changed series 

 pull ahead immediately the first period closes, while during the rest of 

 the year the plants either do not begin to take advantage of the full 

 su])ply in the one case or else they do not feel the lack of the full supply 

 in the other till a later date. 



The maximum amount of growth that can be made with a given 

 constant supply of nutrients before growth ceases varies with the time 

 of year, i.e. with the environmental conditions. Plants grown during 

 May and June averaged 26 gms. dry matter and the total ranged down 

 to 14 or 15 gms. in plants grown in spring or autumn. The winter grown 

 plants did not reach the stage of ceasing to grow, but judging by the 

 trend of the efficiency index curve it is probable that the dry weight 

 would have been below that of the spring and autumn plants. The 

 reason for this variation in dry matter produced is probably that growth 

 is not only dependent upon the mineral food supplied to the root but 

 also on the quantity of carbohydrates produced by assimilation. Under 

 favourable conditions of temperature and sunlight when assimilation is 

 greatly encouraged the formation of carbohydrate is very rapid and the 

 amount of other constituents of dry matter become proportionately 

 though not actually less. This is illustrated in an exaggerated form by 

 those plants which store up starch or sugar, as potato and beet, but it 

 holds good in other green plants as well. When conditions are less 

 favourable to assimilation less carbohydrate is produced in proportion 

 to the amount of food material absorbed, and with a limited supply of 

 the latter a lower total growth is produced. 



One curious discrepancy between the changed and unchanged series 

 is noticeable. The maximum growth with unlimited food supply was 

 made in a series grown from March to May, but in another grown from 

 May to July there was a decided falling oil which was attributed to less 

 favourable environmental conditions. With limited food, however, the 

 maximum growth was made in the second set, from May to July, so that 

 apparently the environmental conditions that are optimum for dry 

 weight production in the presence of plenty of food are not necessarily 

 the optimum when a restricted supply is available. 



