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Introduction. [BOOK m.

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 vonMohl 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