Quantitative Analysis of the Growth o/Helianthus annuus. 371 



larger plants the carbon dioxide was absorbed by NaOH in Eeiset towers. 

 The plants were cut off at the level of the ground. The plants used for 

 respiration determinations were gathered towards the end of the day so that 

 they should be taken after a period of active assimilation. Sufficient time 

 was allowed for the tissue to cool down to the temperature of the respiration 

 chamber. Whilst the plants were cooling down air deprived of carbon 

 dioxide were drawn over at the same rate as that at which it was to be 

 carried over during the experiment. The vessels used were always as small 

 as the size of the material would permit in order that the time interval 

 between the evolution of carbon dioxide from the tissues and its absorption 

 by the baryta or soda should be minimal and that the current necessary to 

 keep the carbon dioxide-content of the air in the vessel low should not be 

 excessive. To this end it was found very suitable to enclose the plant 

 material in a flexible air-proof fabric, since a covering of such a fabric 

 collapses when air is drawn through it and clings tightly to the plant 

 material. Until the plants were about 10 grm. in weight they were used! 

 entire, but subsequently the respiration of the stem, leaves and flowers was- 

 measured separately. 



4. Experiments to Determine the Belation between Temperature and Bespira- 

 tion, and a Suitable Temperature at which to Determine the Respiratory 

 Index. 



It is known that at high temperatures the rate of respiration of plant 

 tissues decreases after a short interval. This falling off in the rate of 

 respiration has been attributed to a decrease in the effective amount of 

 respiring cell-matter due to the high temperature (" time factor " of 

 Blackman). The result may, nevertheless, in some cases be due to a 

 decrease in concentration of respirable material with time. At medium and 

 low temperatures it has been shown by Kuyper (5) for seedlings, and in 

 this laboratory for leaves, that the respiration remains constant for an 

 appreciable interval of time after separating a tissue from its source of 

 carbohydrate supply. 



With these considerations in view, a series of preliminary experiments 

 was conducted to investigate the respiration of cut plants of Eelianthus at 

 various temperatures in order to ascertain for what length of time, after 

 gathering, the respiration remained constant. The results provide at the 

 same time data for the construction of a temperature-respiration curve. 



Experiments were carried out with plants while young, i.e., before the 

 fourth pair of leaves had begun to develop. During this period, as will be 

 shown later, the respiratory index changes little with age. Constant. 



