20 PLANT-LIFE 



score of other ways we may learn the truth that amounts 

 to a botanical axiom no sunlight, no chlorophyll. 

 Moreover, it is demonstrable that this pigment occurs 

 only in parts of plants to which sunlight penetrates; 

 in the cells of the interiors of plants, which sunlight 

 cannot reach, there is no chlorophyll, as we can easily 

 realize when we cut a twig of a tree transversely, and 

 notice that the major portion of the section is not green, 

 and that the green of the twig is confined to its circum- 

 ference. Garden celery is bleached by being " earthed- 

 up." In order that chlorophyll may be formed in the 

 cells of a plant, it is essential that they should contain a 

 small modicum of some salt of iron in solution. Plants 

 grown in solutions from which iron has been purposely 

 omitted are affected with chlorosis; they are sickly and 

 pallid. A very minute quantity of an iron salt added 

 to the water will quickly remedy the trouble, for then 

 chlorophyll is rapidly formed, in the presence of light, 

 and performs its important function in plant-economy. 



But why does the potato-shoot produced in the dark 

 die when it has exhausted the reserves of food in the 

 tuber ?y Or, to put the obverse question, why does the 

 shoot growing in sunlight continue to grow after all re- 

 serves in the tuber have been used up ? In the former 

 instance no chlorophyll is developed, while in the latter 

 it develops in abundance. The conclusion is that 

 chlorophyll is most intimately associated with plant 

 nutrition; that in its absence plants cannot assimilate 

 essential food elements. Now, it happens that about 

 half the solid substance of plants consists of one element, 

 carbon; this is a sine qua non of plant existence, and, 

 without the slightest doubt, the carbon required by 



