PREFACE. 



THE appreciation shown toward the translation of Gels' 

 Pflanzenphysiologische Versuche prepared by the present writer, 

 together with the comments and suggestions from laboratories 

 in which it has been used, has led to the preparation of this 

 manual, which it is hoped will conform somewhat more nearly 

 to the needs of American students. The general form of 

 Gels' manual has been retained, and many cuts from the 

 translation and a few paragraphs of the text have been re- 

 peated here without indication of their origin. 



Only the more important and better established portions of 

 the subject are treated, and these in the manner already in gen- 

 eral use. With the rapid advance of investigation it is next to 

 impossible that an elementary laboratory manual should include 

 the latest results, especially when the essential points of many 

 of them may yet be in controversy and need the critical treat- 

 ment which is certainly not within the province of a work of 

 this character. In the hands of an instructor in touch with 

 current botanical thought, such deficiencies are easily supplied. 



In the interests of precision, the term " assimilation " is 

 here used exclusively to denote a general function of proto- 

 plasm, while the term " photosynthesis," which was introduced 

 into the translation of Oels' manual (Preface and page 30), is 

 adopted to signify the special process of forming carbohydrates 

 from carbon dioxide and water in the presence of chlorophyll 

 and sunlight. 



The author is indebted to Mr. R. N. Day and Miss J. E. 

 Tilden for the demonstration and drawing for Figure 32. 



D. T. MACD. 



MINNEAPOLIS, MINN., April 15, 1895. 



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