PLANT PHY8I0L0OT. 95 



vegetation came originally from chlorophyll-bearing cells, 

 made active by the light. Just how the light affects the 

 chloroplasts in carbon-assimilation is not known, nor do 

 we know how light brings about the formation of chloro- 

 phyll by the protoplasm. We can only regard light as a 

 force which, acting upon the complex compound, proto- 

 plasm, produces molecular changes resulting in the secre- 

 tion, first, of chlorophyll and, second, of a carbon compound. 

 Here it must be remarked that not all cells secrete chloro- 

 phyll in the light, although many which are normally 

 colorless become green under its influence; thus, while 

 many roots and underground stems become green on ex- 

 posure to the light, the petals of many flowers, the stems 

 of the dodders, and the cells of fungi when so exposed 

 develop no chlorophyll. It is a fact, however, that some 

 kind of coloring-matter is produced in nearly all cells on 

 exposure to the light, as is well shown by the familiar 

 experiment of growing flowers, fruits, and various fungi in 

 complete darkness, when they are usually much paler or 

 wholly wanting in color. The color of some flowers 

 appears to be independent of the direct action of light, as 

 shown by Sachs, who obtained perfectly normal flowers of 

 the tulip, iris, squash, and morning-glory when grown in 

 the darkness, although the leaves were completely etio- 

 lated. 



169. Light appears to be essential to plants only as 

 enabling them to assimilate carbon; therefore those which 

 get their carbohydrates from others can live in total dark- 

 ness. Thus many saprophytes (i.e., plants which live 

 upon dead or decaying vegetable matter) are found in dark 

 cellars, caves, mines, etc., growing to full size and matur- 

 ing their fruits perfectly. So, too, some parasites (i.e.. 



