PHOTOSYNTHESIS 79 



1. PHOTOSYNTHESIS. 



Observation of the plant kingdom shows that plants fall 

 into two primary physiological classes. The first includes the 

 familiar green plants. These form the great bulk of vegeta- 

 tion, and embrace those kinds which best exhibit the typical 

 and characteristic plant form, structure, and habit; they all sub- 

 sist upon materials which they take from the earth and the air. 

 The second class includes the parasites and saprophytes. These 

 are not green, are insignificant in bulk, are dependent for their 

 subsistence upon other organisms either living or dead, and are 

 mostly the degenerate descendants of green plants. It is evi- 

 dent, therefore, that the green color, or Chlorophyl, is correlated 

 with the dominant, the typical, even the ideal state of plant life, 

 while its possession constitutes the most distinctive feature of 

 plants. From these considerations it is obvious that Chlorophyl 

 must have some meaning of fundamental importance in the 

 physiology of plants, and this meaning the student must now 

 proceed to determine. Logically the first problem presented 

 is the following: 



What are the essential facts about Chlorophyl, especially its 

 exact mode of occurrence, its physical and chemical nature, and its 

 properties? 



This may be determined first (unless the matter be already familiar 

 to the student from his earlier studies) by exact observation, neces- 

 sarily microscopic, of its occurrence in favorable living tissues, and 

 second by a study of its various optical and other qualities, which may 

 most conveniently be made upon a solution of the substance in alcohol. 



Observation. Select a very thin living green leaf, such as that 

 of a moss, fern prothallus, or Elodea; mount it in water for the micro- 

 scope, and note the appearance, location, and other visible features 

 of the chlorophyl, expressing the results of observation in colored and 

 annotated drawings. Later extend your knowledge of this subject 

 by study in the best accessible books. 



Experiment. Select the best available of the materials mentioned 

 below ; place about 100 sq. cm. of the leaf in a loosely stoppered test- 

 tube or conical flask, with about 60 cc. of alcohol, warm in a water- 

 bath kept at 5o°-SS° until the leaves are blanched (which requires but 

 a few minutes); pour the solution into a clean test-tube, and (a) 



