94 BIOLOGY. [BOOK n. 



chlorophyllian substance in a great measure escapes destruction 

 in the perennial plants. It re-dissolves along with the starch 

 which it englobes, and the whole, passing through the petiole, 

 and carrying along even the phosphoric acid and the potash, 

 wanders toward the permanent organs of the plant. 1 



Before we speak of the special properties of chlorophyll, it is 

 opportune to signalise the importance of the metallic element 

 which it contains. The atoms of iron which enter into its com- 

 position constitute in effect an integrant part of it ; without 

 them it is not endowed with its special properties. Another metal, 

 potassium, though not figuring in the complex molecule of the 

 chlorophyll, seems to play an important part in its nutrition. 

 When the plant does not absorb chlorure of potassium, or at least 

 nitrate of potash, the particles of chlorophyll have less vitality 

 and are incapable of forming starch. 



We have succinctly described the morphological evolution of 

 chlorophyll. It remains for us now to speak of its function. 



Priestley was the first to observe that the green parts of 

 plants exhale oxygen. He put under a receiver in confined air, 

 where mice had died asphyxiated, some mint plants, which lived 

 and flourished energetically there. The chlorophyllian property 

 was thus discovered ; but it was Ingenhouz who attributed the 

 disengagement of vital air operated by plants to its true cause, 

 the action of light. 2 The same observer demonstrated also the 

 inverse action of the flowers and of the roots, which night and 

 day exhale carbonic acid and vitiate the atmosphere. 



Every terrestrial or aquatic plant furnished with chlorophyllian 

 cells, and exposed to the solar light, borrows from the air carbonic 

 acid, and restores to the air an equivalent volume of oxygen. 



For most plants, the activity of the phenomenon is in proportion 

 to the intensity of the light. Though the chlorophyllian function 

 has need in almost all plants of full light, it still exercises: 

 itself, feebly indeed, in diminished light, and there are even 



1 J. Sachs, Traite de Botanique. 



2 Ingenhouz, Experiences sur les Vegetaux, 1780. 



