536 



PLANT GROWTH AND PLANT COMMUNITIES 



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Light Level ( daylight = i o) 



Figure 3. The influence of shading on the net assimilation rate (N.A.R.), 

 the leaf-area ratio (L.A.R. ), and the relative growth rate (R.G.R.) of Medi- 

 cago saliva and Endymion non-script us. 



maximal in full daylight. Another point to be noted is the slow growth 

 of E. non-scriptus (less than 2 per cent per day in full daylight); it is 

 apparent that the major component responsible is the low leaf-area 

 ratio-23 to 28 cmVg. 



This lack of plasticity in E. non-scriptus contrasts with the be- 

 havior of Geum urboninn, a species associated with shady habitats in 

 Great Britain (see Figure 4). A comparison of Figure 4a with Figures 

 2 and 3 shows that shading augments the leaf-area ratio of this plant 

 much more than that of the other species, while the depression in the 

 net assimilation rate follows a pattern similar to that of the others. Thus 

 it is the high degree of plasticity in the leaf-area ratio that is largely re- 

 sponsible for the fact that the plant attains its maximal relative growth 

 at about one-half of full daylight. 



G. iirbanum does not conform to the concept of an obligate shade 

 plant in which the rate of photosynthesis reaches a maximal value at 

 intensities well below those ruling in open habitats. However, Good- 

 all's (1955) studies of the reactions of young plants of cocoa {Theo- 

 broma cacao ) provide an example of a species where shading has little 

 effect on the net assimilation rate ( Figure 4b ) . Thus in this instance it 



