260 WILLIAM E. LAWRENCE 



be encountered with different developmental stages, as in case 

 of the different seasons of the year for all plants and of different 

 years for biennials and plants of a more than two year cycle. It 

 should be possible to construct charts to suit the peculiar physio- 

 logical characteristics of any given plant, as in the case of special 

 researches. 



Factors bearing upon assimilation may be worked out by fol- 

 lowing the arrows back to their origin. Catabolic processes may 

 be traced from protoplasmic decomposition through respiration 

 and other decomposition processes. These reactions are charac- 

 terized by the release of energj^ and also by the production of a 

 great variety of substances, such as chlorophyll, enzymes, fats, 

 and the whole category of so-called by-products such as essential 

 oils, gums, resins, organic acids, tannins, alkaloids, pigments, cell 

 walls, etc. Thus is brought out, with considerable clearness, the 

 whole chemical cycle of the plant, which ends with the ultimate 

 return of all chemical elements and energy to their original source, 

 the surroundings. 



In conclusion, emphasis should be given to the pedagogical 

 importance of building up this scheme before the student's eyes 

 from day to day. It is essential that each succeeding lecture 

 should take up the uncompleted chart at the point to which it 

 was previously carried, until, finally, the whol^ life-C3^cle has been 

 traced. 



The writer wishes especially to express his thanks to Prof. Bur- 

 ton E. Livingston, of the Johns Hopkins University, for his inter- 

 est and criticisms; and to all others who have kindly given helpful 

 suggestions. 



ADDITIONAL NOTES AND EXPLANATIONS 



1, 2, and the water produced in the process of respiration constitute the so-called 

 "metabolic water." See: Babcock, S. M. Metabolic water: Its production and 

 role in vital phenomena. Univ. of Wise. Research Bull. 22: 87-181, 1912. 



3. Elements not used in protein manufacture but set free in the process. 



4. Includes such substances as creatin, and creatinine, as well as CO2. See: 

 Sullivan, M. X. The origin of creatinine in soils. U. S. Dept. Agric, Bureau of 

 Soils, Bull. 83, 1911. 



5. The portion of the chart within the heavy dotted line includes the reactions 

 that go on within the plant body. 



