§ 1] UPON THE RATE AND QUANTITY OF GROWTH 353 



figure shows plainly how, in general, an increment or a decre- 

 ment in one of these quantities is accompanied by a corre- 

 sponding change in the other. 



The cause of this relation between changes in volume and in 

 moisture is partially explained by considering the quickness 

 with which increased growth follows increased moisture. It 

 is undoubtedly due, as Tschaplowitz ('86) hast suggested, to 

 the diminution in the transpiration of the plant in moist air as 

 compared with dry air. The change in the rate of transpira- 

 tion is, however, not to be conceived as an immediate physical 

 result of the change in moisture, but as a response to the stim- 

 ulus of greater or less water in the atmosphere. 



The amount of water in the soil also has an important influ- 

 ence on the rate of growth. Quantitative studies on this well- 

 kuown fact were afforded by Hbllkiegel, who reared barley 

 in soils which contained various fractional parts of the satura- 

 tion quantity. Giving his results in the form of a table, we 

 have the following relation between the humidity of the soil 

 and the amount of dry matter produced, after a certain number 

 of days, in the grain and in the chaff : — 



TABLE XXXVII 



The principal conclusion that one can draw from this table is 

 that there is an optimum humidity of the soil for growth, 

 which is not, however, the same for all organs. 



INIore extensive researches upon this subject have been made 

 by JiTMELLE ('89), who studied chiefly the eifect of water 

 upon the growth of the various organs of the plant, and by 



2a 



