534 PLANT GROWTH AND PLANT COMMUNITIES 



throughout the year under glasshouse conditions. To do this, he rigor- 

 ously selected for each experiment a batch of plants each of which had 

 eight leaves, and on 24 occasions during twelve months he measured 

 the changes in root, stem, and leaf weights over a period of 24 hours. 

 It was found that the relative growth rate of the whole plant ranged 

 from a negative value in December to a maximum of 24.7 per cent per 

 day at the height of summer, and that the changes in the net assimila- 

 tion rate ( calculated on a leaf-weight basis ) closely followed the varia- 

 tions in relative growth rate, while during the summer months the leaf- 

 weight ratio fluctuated within narrow limits. During daylight the light 

 intensity was measured at 30-minute intervals, and a statistical ap- 

 praisal was made of the effects of light, the mean day and night tem- 

 peratures, and the saturation deficit of the air on the diurnal gain in 

 dry weight as a percentage of the initial leaf weight. The multiple 

 regression incorporating all these variables was highly significant, but 

 when further analysis was undertaken to separate the effects of five 

 environmental factors, only the level of light intensity was significant. 

 In passing, it should be noted that during the summer months the glass 

 of the greenhouse was covered with "summer cloud," and that the time 

 from sowing to the eighth leaf stage varied greatly— i.e., from eleven 

 weeks in midwinter to four weeks in midsummer. 



The influence of shading on vegetation development 



Since seasonal changes in light and temperature are themselves 

 highly correlated, the separation of their effects is rendered difficult. 

 As a first approach, it may be more rewarding to conduct experiments 

 in which only one factor is operating, and the major effects of the light 

 factor can be elucidated by conducting shading experiments in which 

 the design is such that the light quality is not altered and the methods 

 of shading bring about only small differences in the air and soil tem- 

 peratures. Research along these lines has long been one of my interests, 

 and some of the results I propose to touch on have already been pub- 

 lished ( Blackman and Rutter, 1947, 1948; Blackman and Wilson, 1951, 

 1954; Blackman, 1957; Blackman and Black, 1959). 



A number of pot experiments have been conducted on the com- 

 parative effects of shading during the summer months on the growth in 

 the early vegetative phase of H. anniiiis and Fogopynim esculentum. 

 A representative set of data is given in Figure 2. It is seen that, irrespec- 

 tive of the degree of shading, the relative growth rate of F. esculentum 

 is higher than that of H. annuiis, because the differences in leaf area 

 ratio in favor of F. esculentum more than offset the higher net assimi- 

 lation rates of H. annuus. It is equally apparent that the order of de- 

 pression of the relative growth rate caused by a given reduction in the 



