Winifred E. Bkenciiley -J-Jo 



the plant, so that at no time is tlie gi-ovvth encouraged by a .spell oi 

 greater heat or more abundant sunshine. As soon as the rate of growth 

 reaches its maximum in the spring and autumn it begins to fall off 

 slowly and irregularly but as at these times of the year growth was not 

 completed within the period of experiment it is not certain what the 

 behaviour would be at the end of growth. In the summer the rate of 

 growth tends to fall away from the maximum but remains very high 

 for several weeks. During this period the plants ])robably reach their 

 maximum efficiency, making the fullest use of the food supplied under 

 favourable conditions of temperature and sunshine. Growth is so rapid 

 that it draws to a close towards the end of the experimental period and 

 this is indicated by a sudden drop in the efficiency index, which oc- 

 casionally becomes negative at the very end, indicating the continuance 

 of respiration after assimilation has ceased, possibly combined with 

 some degree of desiccation. The maintenance of a high rate of growth 

 for several weeks during the summer combined with a sudden later drop 

 is in marked contrast to the slow steady fall in the earlier part of the 

 year, and explains fully why the later sown plants are so much more 

 weighty than the rest. The later fall in the indices is not determined by 

 either temperature or sunshine, but is a characteristic feature in the 

 physiology of the plant occurring towards the end of growth, but the 

 degree in which the fall is marked does depend to some extent upon the 

 environmental conditions, as it is more obvious when favourable cir- 

 cumstances have kept up a rapid rate of growth until late in the life 

 of the plant. The importance of the rapid rise to the maximum rate of 

 growth is well shown by series K and M, both grown in the summer 

 months. M reached a considerably higher maximum efficiency index 

 than K, but whereas in K the rate of growth was at its maximum within 

 a week of the 1st period, in M three weeks elapsed before this point was 

 reached. Consequently, in spite of the ultimate rate being higher in 

 M, the total growth was only 71 gms. against 91 gms. in K. 



In the July series the eft'ect of the excessive heat and insolation 

 was not manifest at first, as the maximum rate of growth was reached 

 at the usual time, one week after the end of the first period. The rate 

 fell off steadily though not abnormally quickly for another three weeks, 

 but by that time the energy of the plants was exhausted and during the 

 next fortnight the efficiency index fell rapidly until at the end the plants 

 were prematurely dead. 



An examination of the data b}^ statistical methods shows a remarkable 

 agreement with the results deduced by observation during growth and 

 comparisons of the figures and curves for the individual series. 



