576 THE HISTORY OF BIOLOGY 



for both the botanists of the romantic school and their immediate successors 

 had mostly applied themselves to morphology. Sachs, on the other hand, 

 conscientiously devoted himself from the beginning to the problem of plant 

 physiology and the means of exploring it. Among his earliest works may 

 be mentioned his investigations into the physiological part played by the 

 chlorophyll granules, which he elucidated in its most essential features: he 

 established the fact that starch formation is the first product of carbonic-acid 

 assimilation, and, further, that sunlight plays the decisive part in this proc- 

 ess. He investigated the different parts of the solar spectrum with the view 

 to discovering their influence upon the alimentation of plants. Again, the 

 continued metabolism and conveyance of nutrient substances within the 

 plant has been worked out by him in all essential features. He studied all 

 the vital processes in the vegetable kingdom with a view to ascertaining 

 their intensity, which he expressed in graphic form. It was mainly, however, 

 the movements of flowers that he systematically investigated; he established 

 the dependence of the direction of growth upon the law of gravity and in- 

 vented a rotating apparatus by means of which this growth can be studied 

 under abnormal conditions. He is responsible for the method of investiga- 

 tion and thorough exploration of all that goes by the name of tropisms — 

 at least as far as the vegetable kingdom is concerned. As a basis for all these 

 phenomena he mentions irritability, a quality which he believes originates 

 only in the living plasm and which he carefully analyses in respect of its 

 various manifestations — an investigation that has been of immense theo- 

 retical importance for subsequent research. Finally, it may be mentioned 

 that Sachs was the finest text-book writer of his time. Through his manuals 

 the facts that he discovered and the ideas that he defended have spread 

 far beyond the circle of his personal pupils. 



Among these pupils Wilhelm Pfeffer (1845-1910) is worthy of spe- 

 cial mention. When still quite young he became a professor, first at Tubingen 

 and afterwards at Leipzig, where he subsequently laboured as a brilliant 

 teacher and investigator. He made a special study of the phenomena of 

 growth in plants and the influence exerted upon them by both external and 

 internal factors. He investigated a number of external influences and made 

 valuable contributions to our knowledge of them, at the same time consid- 

 erably improving the technique employed for their study. However, he 

 emphatically declares that external influences do not act directly in a de- 

 velopmental way, but only cause a change of activity in the plant itself. 

 He sees in the study of this reciprocal action between external influences 

 and internal manifestations of life the very aim of biology, and '' dkjormative 

 Determinierung der Zellen und der Organe ' ' becomes the object of his close analy- 

 sis. In this study very careful attention is paid to all co-operating factors, 

 so far as they can be calculated, and unwarranted attempts at simplification 



