220 Quantitative Analysis of Plant Growth 



obtained by Weber by the "growth" method, but are distinctly lower 

 than those obtained by the present writers by the same method. 

 Weber's experiments, however, are open to criticism on the ground that 

 his plants were grown in pots, under which condition Helianthus does 

 not flourish, and moreover, they were grown in a greenhouse where 

 the light would be considerably less than that under natural conditions. 

 Our value 8-5 is based on experiments carried out under natural 

 conditions. For a certain week, i.e., the fourth from germination, the 

 Unit Leaf Rate was found to be 9-0. During this week the respiration 

 was measured continuously. When allowance is made for the loss in 

 dry-weight due to respiration the value for assimilation becomes 

 12-3. The real value would probably be slightly higher since the 

 respiration of a plant exposed to the direct rays of the sun would be 

 higher on account of increased temperature — the temperature of our 

 respiration experiments was the shade temperature. The hours of light 

 for this week numbered 116 1 . Utilising this figure for the hours of light 

 we obtain a value of 8-5 mgs. C0 2 per 50 cm. 2 per hour of light. This 

 value 8-5 includes salt-uptake, which at the most would not be more than 

 7 per cent., thus it is shown definitely, for Helianthus, that the values 

 estimated by the "gasometric" method, which moreover does not include 

 the hours of faint light in the earlier part of the morning and in the later 

 part of the evening, do not give a reliable estimate of the assimilation 

 which the plant can carry out under natural conditions. Our figure of 

 8-5 is smaller than the figure obtained by the "half-leaf" method. 



We propose to consider on a later occasion the probable reason for 

 the "growth" method giving lower values than does the "half -leaf" 

 method. 



V. Summary. 



In this chapter we have continued our analysis of the results of the 

 experiments on the growth of maize carried out by Kreusler and his 

 co-workers. The rate of growth has been expressed per unit leaf-area 

 instead of per unit dry- weight as in the last chapter. The term "Unit 

 Leaf Rate " is used for the weekly rate of increase of dry-weight in mgs. per 

 sq. cm. The Unit Leaf Rate, instead of undergoing a perfectly definite 

 type of variation, as does the Relative Growth Rate, fluctuates about a 

 mean value. The larger fluctuations which occur in the values for Unit 



1 If it is assumed that light below 1/3 sunlight is limiting, the hums of light become 

 88 and the value for assimilation 11-2. 



