96 PLANT PHYSIOLOGY 



respiration and a gain from absorption of minerals, will be approximately 

 the same in both. 



The foregoing experiments will make it plain to the student 

 that he is here concerned with a process of formation, or syn- 

 thesis, of a substance under the action of light, a process appro- 

 priately known as Photosynthesis, while the substance made 

 may appropriately be termed the Photosynthate. 



If he experiments at all widely upon starch formation, the 

 student cannot fail to observe that different plants give the 

 starch test with very different degrees of readiness, some of 

 them giving none at all. It becomes now a question of great 

 interest whether the latter plants increase in weight, a matter 

 which the student should determine by experiment if condi- 

 tions allow, otherwise from the literature. 



Suggested Experiment. Using the general method of page 92, but apply- 

 ing it to some plant which shows no starch (an Allium, Scilla, or some other in 

 the list by Meyer, Botanische Zeitung, 1885, synopsis in Jost, hi), ascer- 

 tain whether there is any increase in weight of the green tissues in light. 



This experiment brings the student face to face with other 

 forms of the photosynthate, and also with the very important 

 subject of its chemical composition. The subject involves 

 problems in organic chemistry rather impracticable for experi- 

 mental study in the present course, but the student must follow 

 it through the literature. In this way he should carefully work 

 out, and express in a clear exposition, the present state of knowl- 

 edge of the various forms of the photosynthate, their chemical compo- 

 sition, the stages in their formation, their interrelationships, and 

 the reasons why starch appears in leaves. He will be greatly 

 aided in the study by some experimentation upon the simpler 

 phases of the subject, on which he can find directions in Detmer, 

 46, and especially in Darwin and Acton, 276. 



These studies will show that the chemical composition of 

 the photosynthate approximates to C 6 H 12 6 , which may be taken 

 as a conventional formula for the photosynthate in general. This 

 at once raises the question as to the source of supply of these 

 elements. Scrutiny of the formula suggests that since the hydro- 



