409] E. 8. Johnston 211 



SEASONAL VARIATIONS IN THE GROWTH-RATES OF 



BUCKWHEAT PLANTS UNDER GREENHOUSE 



CONDITIONS 



By EAEL S. JOHNSTON 



Seasonal variations in greenhouse plants are of considerable 

 importance to plant growers as well as to experimenters in 

 plant physiology,, but it is especially with reference to physi- 

 ological experimentation that this study was undertaken. 

 When it is necessary to repeat an experiment on plant growth 

 it often occurs that the results of the second experiment are 

 in more or less pronounced disagreement with those of the 

 first. Since the controlled external conditions must be re- 

 garded as the same- for both experiments, such disagreement 

 appears to be related either to initial differences in the plants 

 used (internal conditions) or to uncontrolled external con- 

 ditions as these vary with the season. The first of these pos- 

 sibilities is probably not as important as the second in most 

 cases, for care is usually taken to select plants for the second 

 experiment that are apparently similar to those used for the 

 first. While this problem of similarity of internal conditions 

 of different lots of plants is a very difficult one and is hardly 

 susceptible of quantitative study at the present time, it is 

 quite possible to carry out studies on the relation of growth 

 to the usually uncontrolled (or only partially controlled) 

 external conditions of a greenhouse., as these conditions 

 change throughout the year. A portion of the results ob- 

 tained from such study are here presented. 



A set of similar water cultures was started every two weeks 

 and each was continued for a period of four weeks, so that 

 the periods of successive sets overlapped. A single set con- 

 sisted of ten plants, each suitably supported in a glass jar con- 

 taining about 425 cc. of nutrient solution. These jars were 

 covered, to exclude most of the light from the plant roots. 

 The solution was renewed at the middle of each four-week 



