u8 



RELATIONS OF PLANTS TO LIGHT 



chlorophyl is broken down faster than it can be built up, consti- 

 tuting a maximum. This maximum must be regarded as a point 

 at which the constructive efforts of protoplasm under the stimula- 

 tion of light are overbalanced by the disintegrating effects which 

 are exhibited by rays from all parts of the spectrum. The in- 

 duced temperatures doubtless play a part in the process. The 

 disintegration of chlorophyl in darkness is 

 probably a reaction on the part of the plant to 

 remove a substance which has become useless 

 and which is maintained at great cost. Some 

 forms, such as the Cactaceae, conifers and 

 ferns, retain the chlorophyl unchanged, how- 

 ever. 1 



181. Formation of Chlorophyl in Darkness. 

 Secure some healthy specimens of Botrychium, 

 Osmunda, or Aspidium 

 or any convenient fern 

 and remove them from 

 the soil, if out of doors 

 about December i st, 

 and set in flower pots of 

 suitable size. Bring into 

 forcing room and dark 

 room gradually. Note 

 general form of etiolated 

 specimens and also the 

 presence of chlorophyl. 

 Examine the chloroplasts and compare with those of the normal 

 individuals. Germinate seeds of Pinus, Thuja or other coniferous 

 trees in soil in dark chamber. 



182. Growth of Green Plants in Darkness, Place a number of 

 nearly mature plants of Narcissus, or Arisaema, in the dark cham- 

 ber and note the behavior of the leaves in regard to growth, and 

 persistence of the chlorophyl. 



i See Pfeffer, W. Plant Physiology, i : 233. 1900. 



56 57 



FlG. 56. Viola obliqua (etiolated). 

 FIG. 57. Viola obliqua (normal). 



