and thus obtain the carbon which plants accumulate in organic
combinations, but that all parts of plants also absorb at all
times smaller quantities of oxygen, and exhale carbon dioxide,
and so perform a process of respiration exactly corresponding to
that of animals. He was soon followed by Theodore de Saussure
with more thorough investigation of these processes, and with
proofs that the ash-constituents of a plant are no chance or unimportant addition to its food, as had been hitherto commonly
supposed (1804). The influence also of general physical forces
on vegetation was established in some important points, though
not yet submitted to searching examination. Thus Senebier
showed in the period between 1780 and 1790 the great effect
which light exercises on the growth and green colour of plants,
and De Candolle at a later date discovered its operation in the
case of leaves and flowers that show periodic movements.
Still more important was Knight's discovery in 1806 that the
upright growth of stems and the downward direction of the
main roots are determined by gravitation.
3. This second period of important discoveries was also followed by a relapse, and again doubts were raised as to the correctness of the very facts which had been best established; attempts were made under the influence of preconceived opinions to invalidate or ignore these facts, and to substitute for them theories that wore the guise of philosophy. The so-called nature-philosophy, which had long been a great hindrance to morphology, proved in like manner injurious to vegetable physiology; the doctrine of the vital force especially stood in the way of every attempt to resolve the phenomena of life into their elementary processes, to discern them as a chain of causes and effects. The ash-constituents of plants, and even their carbon, were traced to this vital force, and misty notions connected with the word polarity were used to explain the direction of growth and much beside. In like manner the influence of the nature-philosophy was brought to bear on the established results of the sexual theory to the destruction of all sound
