RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 535 



(a) N.A.R. (b) L.A.R. (c) R.G.R. 



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Light Level ( daylight = i o) 



Figure 2. The influence of shading on (a) the net assimilation rate, (b) the 

 leaf-area ratio, and (c) the relative growth rate of Helianthus annuus and 

 Fagopyrum esculentiim. 



light level is the resultant of a decrease in the net assimilation rate and 

 an increase in the leaf-area ratio. The further conclusion that can be 

 drawn is that the assimilation rate and the growth rate of unshaded 

 plants were restricted by the radiant energy received during August, 

 though it was high enough to allow daily gains in dry weight of 16.6 

 and 15 per cent per day. 



During the course of these investigations, the response to shading 

 of 22 species has been examined. There are good grounds for conclud- 

 ing that for many of the species the relative growth rate of unshaded 

 plants is restricted by the level of diurnal radiation received during 

 the summer months. This light limitation may also apply to species 

 which are common in woodland habitats. As an example. Figure 3 

 shows the reactions to shading of Endymion non-scriptus, a bulbous 

 perennial, and of Medicago safivo, which, it is concluded, has a partic- 

 ularly high light requirement. Since the experimental periods were 

 difiFerent (April as against August) only general comparisons can be 

 made. For both species the reduction in the net assimilation rate with 

 increased shading is of the same order, but the rise in the leaf-area 

 ratio for E. non-scriptus is much smaller. In this species the leaf primor- 

 dia are laid down in the previous autumn, and the influence of light 

 cannot operate until the expanding and extending leaves have emerged 

 above ground in the spring; thus this limits their plastic response. 



Since for both species the depressions in the assimilation rate are 

 not matched by the gains in the leaf-area ratio, the growth rates are 



