76 The Science of Life. 



functions of plants. Sachs was the first to show that 

 the starch which Von Mohl and others had recognized 

 as almost universal in the chlorophyll grains (or chloro- 

 plasts), is the first visible product of elaboration by the 

 chloroplasts under the action of light, and that it passes 

 from its seat of formation to growing and storing tissues. 

 It may be said that no small part of his life-work was 

 concerned with starch and allied substances. In general 

 terms, he devoted himself to the micro-chemical study 

 of the active tissues, a method now farrfiliar, but when 

 Sachs began his work, quite novel. He applied it in 

 particular to the internal phenomena of germination, 

 and to the movements and changes of formed materials 

 within the plant. 



(b) Environmental Stimuli. Sachs made equally great 

 advances by his researches on the reactions of plants to 

 external stimuli. He defined the optimum temperature 

 for germination, studied the heat-rigor and cold-rigor 

 of sensitive organs, showed that heat as well as light is 

 necessary for the formation of chlorophyll, and analysed 

 the various influences of light, and of some rays in 

 particular. By his investigations of the reactions which 

 occur in response to the stimuli of gravity, light, and 

 moisture he placed the study of the irritability of plants 

 on a secure basis. 



(c) Methods. His great manual dexterity and in- 

 genuity of device enabled him to do exact work with 

 very simple instruments, and some of his appliances are 

 now familiar in the botanical laboratory. He made the 

 first growth-measurer (auxanometer) ; he devised the 

 simple "hanging-sieve", with which he studied reaction 

 to moisture; and he introduced the " klinostat ", for 

 studying the reactions of growing parts to gravity. In 

 connection with methods, we may also notice that he 

 gave great attention to the culture of plants in artificial 

 nutrient solutions, a method begun by Duhamel (1758), 

 and of great importance in the determination of the 

 relative physiological value of the different mineral 

 constituents in the plant's food. Sachs also devised 

 the " Lithium -method" of studying the rate of the 

 passage of water and salts up the stem. 



