RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 533 



To sum up the efiFects of the nutrient level: Where increases in the 

 nitrogen supply augmented the relative growth rate, this could be at- 

 tributed to an increase in the leaf-weight ratio rather than to a change 

 in the net assimilation rate on a leaf-weight basis. On the other hand, 

 phosphorus deficiency depressed the relative growth rate by diminish- 

 ing both the leaf-weight ratio and the net assimilation rate. The find- 

 ings of more recent investigations on an extended range of species 

 confirm that similar effects are induced by lack of phosphorus and ni- 

 trogen, save that when the deficiency of nitrogen is extreme, the net 

 assimilation rate can be depressed ( Thurston, 1959 ) . 



Further evidence for the relative constancy of the net assimilation 

 rate on an area basis for sugar beet and mangolds is provided by the 

 data of Watson and Baptiste (1938) from the Rothamsted Experi- 

 mental Station. They sowed both these crops at weekly intervals be- 

 tween April 9 and June 18, and when the net assimilation rates were 

 determined from the end of July onward, no significant effect of the 

 sowing date was recorded. However, since the standard distance be- 

 tween the rows was 56 centimeters and the plants were spaced 23 

 centimeters apart in the row, a complicating factor was present: the 

 differences in sowing dates would result in differential effects of self- 

 shading within and between the rows. 



A notable feature of this investigation was that it was the first 

 occasion on which a detailed growth analysis of the relative perform- 

 ance of two species in the field had been made. Over all the sampling 

 occasions there were no major differences between species in the leaf- 

 area ratio, the net assimilation rate, or the relative growth rate. Be- 

 tween July and November there was a general fall for each of these 

 criteria, but these trends were different from the changes in the size of 

 leaf, the ratio of leaf area to leaf weight, or the rate of leaf production. 

 Here there were significant differences between species. The mangold 

 plants had larger leaves and a greater surface per unit weight of 

 lamina, while the sugar beet possessed a higher rate of leaf production. 



These authors failed to establish any significant effects of either 

 the total radiation per day or the mean daily temperature on the mean 

 value of the net assimilation rate, but they were able to demonstrate 

 that the rate of leaf production was temperature-dependent. On gen- 

 eral grounds they also reached the conclusion that the changes in the 

 ratio of leaf area to leaf weight were more dependent on external than 

 on internal factors. 



From what has been stated so far it is apparent that the diversity 

 of conclusions reached concerning the influence of light and tempera- 

 ture on growth was linked with the difficulty of eliminating age effects. 

 The first attempt to get round this source of error experimentally was 

 made by Goodall (1945) in his study of the growth of the tomato 



