RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 



539 



The changes in leaf-area ratio can be further analyzed, since on 

 consideration it is evident that this ratio is the product of the leaf- 

 weight ratio and the mean of the area-to-weight ratios of the individual 

 leaves. Therefore the observed effects of shading on the leaf-area ratio 

 can be further subdivided into the eflFects on the two components. 

 Where shading has induced only small changes in the leaf-weight ratio 

 {e.g., in H. annuus, F. esculenfum, and L. marifitniis), it would be ex- 

 pected that the variation in leaf-area ratio would be closely linked with 

 the changes in the area-to-weight ratio of the lamina. That this proves 

 to be so is apparent in Figure 7. It is also apparent from Figure 5, 6, 

 and 7 that for barley and G. iirhanum the increase in the leaf-area 

 ratio induced by a diminution in the light level is more dependent on 

 the plasticity of the leaf lamina than on the leaf-weight ratio. 



Mention has already been made of the fact that it has not always 

 been appreciated how rapid may be the plastic responses of plants to 

 fluctuations in the environment. The rapidity of the response of the 

 lamina is illustrated in experiments where H. annuus is transferred 

 from one light level to another. A full account has already been given 

 by Blackman and Wilson ( 1954 ) , and only one experiment will be 

 cited here. Plants of H. annuus in the early vegetative phase were first 

 subjected to 1.0, 0.5, and 0.24 daylight, and then the pots were redistrib- 

 uted in the nine possible combinations of this range of light levels 



en 



e 



u 



o 

 a. 



400 O- 



. 300 



- 2 00 



'^Urn 



'^^^^''^nthus 



■-0 



-o 



lOO 



Leaf Area to Leaf Weight o 

 400 Pl'^rit „ 



Oo 



. 200 



•^* •- 



OS 



I o o 



OS 



I o 



Light Level (daylight= i o ) 



Figure 7. The influence of shading on the ratio of leaf area to leaf weight 

 and the leaf-area ratio of Fagopyntm esculentum, Helianthiis annuus, Hor- 

 deum vulgare, Lathyrus maritimus, and Geum urbanum. 



