RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 537 



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Figure 4. The influence of shading on the net assimilation rate, leaf-area 

 ratio, and relative growth rate of (a) Geum iirbanum and (b) Theobroma 

 cacao. (Data of Goodall, 1955.) 



is the increase in the leaf-area ratio that determines that the relative 

 growth rate is greatest below 0.5 daylight. 



Shading also brings about differential changes in the growth of the 

 root, shoot, and leaves, and these changes lead to alterations in the 

 ratios of the weights of the root, shoot, and leaves to the total plant 

 weight. The results of an experiment in which Avena sativa, H. annuus, 

 and F. esculentum were compared are shown in Figure 5. The leaf- 

 weight ratio of H. annuus and F. esculentum is little affected by a re- 

 duction to 0.24 daylight, but for A. sativa there is a rise. Shading to a 

 small extent increases the stem-weight ratio but depresses the root- 

 weight ratio. 



A marked contrast in the reactions of different species is exhibited 

 by the trends for Lathy rus maritimus, a littoral species which in North- 

 east Europe inhabits shingle banks, and G. urbanum (Figure 6). For 

 L. maritimus, down to 0.24 daylight there are no significant changes 

 in any of the ratios and only small rises or falls at 0.055 daylight. On 

 the other hand, the pattern of readjustment for G. urbanum involves 

 a sharp reduction in the root-weight ratio and some gains in the ratios 

 for leaf and shoot. 



