20 A History of Botany, 1860-1900 



in the plant by the living substance was difficult to under- 

 stand till botanists — Sachs especially — put forward the 

 great generalization of the continuity of the protoplasm 

 through the cell-walls, a theory which passed from the 

 realm of hypothesis to that of fact under the careful 

 investigations of Gardiner and of Russow. The similarity 

 of the principles underlying the construction of animals and 

 plants followed this demonstration, and researches carried 

 out in the later years of the century established the fact 

 of the essential identity of all living substance. Nothing 

 did more than this discovery to clear up all the old con- 

 fusion as to the true nature of the respiration of plants. 

 It led, moreover, to the important elucidation of the true 

 nature of the food of plants, and the recognition that the 

 action of chlorophyll is not the digestive process of the 

 vegetable organism, but that the chloroplast is a distinct 

 mechanism for the primary construction of organic material 

 destined to become the food of animal and plant alike. 

 The details of the metabolic processes in both kingdoms of 

 Nature have since been ascertained to be fundamentally 

 the same, and the action alike of the protoplasm itself and 

 the enzymes it secretes has been shown to be strictly 

 comparable in the two cases. 



In the field of what has been very generally called 

 irritability, but which many physiologists are coming to 

 speak of preferably as sensitivity, perhaps the greatest 

 advances have been made. Prior to i860, but little accu- 

 rate knowledge of this subject had been acquired. The old 

 Aristotelian dogma of the soul of plants, and the nature- 

 philosophy associated therewith, was abandoned nearly 

 twenty years before. De Candolle recognized something 

 of the power of appreciation of the surroundings when 

 he wrote of excitability, though the term hardly expresses 

 the ideas held to-day. Meyen also observed what he 

 called voluntary movements in Hedysarum, and noted 



