2 8S 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



tinuously illuminated plants, although they contained more chlorophyll, pos- 

 sessed a much simpler anatomical structure than the others, and resembled in 

 certain respects, plants grown in continuous darkness. The leaves of Helle- 

 borus niger, for example, had normal structures when the plants were darkened 



every night; the mesophyll com- 

 prised the usual layer of palisade 

 parenchyma above (containing 

 most of the chloroplasts) with 

 loose, spongy parenchyma below, 

 the latter having numerous large 

 air passages (Fig. 138, /). On 

 the other hand, the Helleborus 

 leaves grown with continuous illu- 

 mination were very different from 

 the others in several respects. 

 Chloroplasts were here much more 

 numerous than in the other case 

 and they occurred almost through- 

 out the entire tissue, instead of 

 being mainly confined to the 

 palisade. Instead of the loose, 

 spongy parenchyma there was a 

 tissue more like the fundamental 

 parenchyma of growing regions, 

 with almost no intercellular spaces at all (Fig. 138, F). 



While photosynthesis is mainly dependent on the less refrangible half of the 

 spectrum, normal growth and development require the more refrangible half 

 (Fig. 132, curve XY). These more refrangible rays (blue and violet light) are 

 strongly absorbed by plants. 

 If, on a bright spring day, for 

 example, the intensity of the 

 blue-violet light is 666 in the 

 open , it is only 2 1 in the shade 

 of a fir tree, all but about one 

 thirty-second of the energy of 

 these rays having been re- 

 flected or absorbed by the 

 leaves of the tree. Many 

 formal characteristics of 

 plants depend upon the intensity of the blue-violet light that reaches them. 

 In evergreen plants, only the peripheral leaf-buds develop, since the interior 

 buds are shaded, but in deciduous trees leaf-buds develop throughout the 

 crown; in the latter case the tree is leafless at the time the buds are opening 

 and all buds are at first equally lighted. 1 



Plants differ with respect to their light requirements and they may be 



1 Wiesner, 1893. [See note 2, p. 283. 1 



Fig. 137. — Upper portion of plant of Campanula 

 rotundifolia, with reniform leaves developed from a 

 lateral bud in diffuse light. (After Goebel.) 



Fig. 138. — Cross-sections of leaves of Helleborus niger, 

 grown in continuous light (F) and darkened during the 

 night hours (J). (After Bonnier.) 



