PLANT PHYSIOLOGY 387 



varied in a similar manner with the same kind of variation in 

 external conditions, whereas the growth-rates measured in 

 terms of stem-elongation varied in another way with the same 

 external differences. The seasonal marches of the growth-rates 

 for Oakland were found to be markedly different from those 

 for Easton. It appeared that temperature was clearly the 

 limiting condition for growth during the first two weeks. 

 During the second two weeks of growth, however, with exactly 

 the same environmental conditions, the moisture relation {i.e. 

 rainfall-evaporation ratio) appeared, in many cases, to have 

 been the limiting condition for growth, this being especially 

 true when the temperature was high. It thus appears that if 

 two plants in different stages or phases of their development 

 are exposed to the same fluctuations in environmental con- 

 ditions, the limiting factor for one plant during a succeeding 

 period may be of an entirely different nature from that for the 

 other. The author points out that this result must be due to 

 a difference between the " internal " conditions of the plants 

 at different stages in development. 



A rather more comprehensive contribution to the study of 

 this problem has been made by F. M. Hildebrandt (" A Phy- 

 siological Study of the Climatic Conditions of Maryland, as 

 Measured by Plant-growth," Physiol. Res., ii, 1921, p. 341), 

 who has presented the results obtained from a study of a series 

 of observations on the climatic complexes for nine different 

 stations in Maryland for the summer of 1 914, as the effectiveness 

 of each complex was automatically integrated by soy-bean 

 plants grown for a period of four weeks from sowing, new seeds 

 being sown every two weeks. Measurements of stem-height, 

 leaf-area, " leaf-product," and dry-weight were taken, the 

 environmental factors studied being the air temperature, the 

 evaporating power of the air, sunlight intensity and duration 

 of sunlight. Indices for total seasonal climatic efficiency 

 derived by multiplying the seasonal average growth-rate per 

 day by the normal length (in days) of the growing season for 

 the station in question, were found to have the following values 

 for the several stations investigated, namely, Oakland, 9,009 ; 

 Chewsville, 12,480 ; College, 16,867; Easton, 17,688 ; Princess 

 Anne, 19,005; Coleman, 21,115; Darlington, 23,688; Balti- 

 more, 25,422. 



Adopting a similar procedure with seedlings of Fagopyrum 

 esculentum, Moench., which were grown for a series of four-week 

 exposure periods over a total time period of thirteen months, 

 Earl S. Johnston (" Seasonal Variations in the Growth-rates 

 of Buckwheat Plants under Greenhouse Conditions," Johns 

 Hopkins Univ. Circ, 191 7, p. 211 ; also "Climatic Conditions 

 in a Greenhouse as Measured by Plant-growth," Monthly 



