WiNiKiiKD Vj. Hrknciii.kv 217 



subjected to rain or wind. Under these conditions the chief variable 

 factors influencing growth were probably 



(1) temperature (maximum and minimum), 



(2) bright sunshine and light intensity, 

 {?)) humidity of the air. 



Of these factors tem))erature records are available throughout. The 

 sunshine figures give a very incomplete idea of the anu)unt of available 

 light or of the intensity of light but unfortunately no ap])aratus for 

 automatically recording the changes in actual light intensity was avail- 

 able, and this part of the investigation is therefore incomplete. The 

 humidity was not measured as at the time the possible significance of 

 change in this respect was not realised, but readings taken during 1919 

 show that the range of humidity within the house is considerable. The 

 results of the investigation are therefore chiefly of use in indicating the 

 relationship of temperature and sunshine to the rate of growth at different 

 periods of the life of the plant. 



Eight series of peas were grown during these observations, the seedlings 

 being placed in water cultures on the following dates: 



"Harbinger."" 



Series B. September 28th, 1915. Series G. January 4th, 191G. 



C. October 28th, 1915. K. March 10th, 191G. 



E. December 2nd, 1915. M. April 2r)th, 1916. 



"King of the Dirarfs.'" 

 0. July 3rd, 1916. Q. October 3rd, 1916. 



When seeds are germinated a loss in dry weight occurs owing to the 

 fact that respiration goes on steadily from the beginning of growth, with 

 a consequent loss of material by oxidation that at this stage is not 

 balanced by the manufacture of fresh plant material. After the seedlings 

 are set up in water culture this loss continues, for the same reason, over 

 a period which varies in length according to the time of year (Figs. 4 

 and 5). Assimilation, which results in the building up of new substances 

 from the air and the absorbed food salts cannot begin till the green leaves 

 have appeared, and even then some little time elapses before the gain from 

 as.similation balances the loss from respiration. Furthermore, the weights 

 obtained show that the gain is at first very slow, as the leaf surface is 

 initially small, and it is often several weeks before the young plant is 

 again as heavy as the seedling was when first put into the culture 

 solution. For the sake of convenience growth is divided into two periods 

 which will be considered separately. 



