PLANT PHYSIOLOGY. 43 



growth, including the variation in the relative weights of 

 root, stem and leaf at different periods in the plant's hfe 

 history, and the effect of fertilisation and flowering on this 

 distribution. It is also necessary to study closely the increase 

 in area of the leaf surface, which constitutes the " catchment 

 area " for the energy derived from sunlight. The respiration 

 of the plant at different periods in its life history must be 

 ascertained, to find out what losses are being sustained in this 

 way. Finally, by burning the various organs of the plants which 

 have attained different stages of development, we can release 

 the " energy " that has been stored up, and can thus follow 

 the process of energy accumulation in the plant throughout its 

 hfe. 



The investigation is being attacked in two ways. In the 

 first place, barley plants growing in the field and in pots at 

 Rothamsted are being studied. In the case of the pot plants,, 

 the dampness of the soil is regulated and kept as constant as 

 possible. Records are kept of temperature and light intensity, 

 and samples of the soil are taken at regular intervals for partial 

 analysis. The gradual increase in leaf area is ascertained by 

 making frequent tracings of each leaf of each plant, and finding 

 the areas with a planimeter. The dry weights of various parts 

 of the plants, such as stem, leaf and root, are ascertained at 

 regular intervals ; part of the dry matter is analysed chemically 

 and part burnt. Most of the operations have to be done on a 

 number of similar plants, so that the amount of work involved 

 is very great. From the data collected during 192 1, a chart has 

 been constructed showing how a plant grows under certain 

 standard conditions. Later it will be possible to compare this 

 with the manner of growth under different conditions, and thus 

 to ascertain exactly the extent to which the efficiency of 

 the plant is altered by changed conditions. By " efficiency " 

 is meant the proportion of the sun's energy that the plant is 

 able to use; this, after all, is the phenomenon on which all 

 living matter depends for its existence. It is important that 

 we should know whether the efficiency of plants growing under 

 different conditions — e.g., light and shade — varies greatly; 

 whether manuring has any effect in increasing the efficiency of 

 the plant ; whether the difference between a light cropper and 

 heavy cropper is a difference in efficiency. 



The other method by which the problem is being attacked is 

 with cucumbers growing under strictly controlled conditions. 

 The cucumber has been chosen partly because it will tolerate 

 great changes in conditions of light, temperature, etc., and partly 

 because it is an important glasshouse crop, grown under condi- 



