374 Introduction. [BOOK in. 



In comparison with this important advance in the sexual 

 theory and the doctrine of the nutrition of plants little was 

 done in the branches of vegetable physiology which remain 

 to be mentioned, and that little appeared in an unconnected 

 and fragmentary state; different observers established the 

 connection between the temperature of plants and oxygen- 

 respiration ; some new single facts were discovered in con- 

 nection with the downward curvature of roots, Briicke published 

 in 1848 an excellent enquiry into the movements of Mimosa- 

 leaves, and Hofmeister showed in 1857 that the phenomenon, 

 then known as bleeding in the vine and some other trees, takes 

 place in all woody plants, and not in spring only but in every 

 period of the year, if the requisite conditions are present. 

 These and many other isolated observations were very valuable 

 for the future, but were not used at the time to frame compre- 

 hensive theories, because no one devoted himself exclusively 

 to questions of the kind with the perseverance, which in these 

 difficult subjects can alone lead to certain results and to a 

 deeper insight into the inner connection of the phenomena. 

 Surprisingly small was the addition to the knowledge of the 

 movement of sap in plants, and still less was discovered 

 respecting the external conditions of processes of growth and 

 the movements connected with them. The important question 

 of the dependence of the phenomena of vegetation on temper- 

 ature, was it is true not wholly neglected ; but the mistake 

 was made of attempting a short cut by multiplying the total 

 period of vegetation of a plant by the mean daily temperature, 

 in the hope of finding in this product an expression for the 

 total warmth required by a given plant ; this mistake was 

 especially misleading in the geography of plants. 



The more valuable knowledge which had been gathered up to 

 1851 was brought together by von Mohl in his often-mentioned 

 work on the vegetable cell with equal perspicuity and con- 

 ciseness, and current views were critically examined ; vegetable 

 physiology generally was expounded at greater length but with 



