460 The Physiology of Plants BOOK in 



more and more light continually manifested itself. The 

 point of view changed rapidly ; movement ceased to be 

 the prominent idea and gave place as such to perception 

 of change ; stimulation as a factor came to be recognized ; 

 the old idea of sensation as an endowment of vegetable 

 substance, held by the older writers in a mistaken form 

 and subsequently denied even by men like Sachs, gradually 

 grew into favour and became established on a firmer basis. 

 Instead of being regarded as the direct and mechanical 

 result of purely physical influences, movement became 

 realized as a vital phenomenon, a result of a response of 

 the plant to some modification of its surroundings. The 

 idea, in fact, of purpose in movement became established. 



The researches which thus led to this result in a term 

 of less than forty years were carried out by many 

 workers in many lands. The earliest, and in some respects 

 the chief place among them must be given to Albert Bern- 

 hard Frank, who published two remarkable contributions 

 to the literature of vegetable physiology in the years 1868 

 and 1870. In the first of these Beitrcige zur Pflanzen- 

 physiologie, he demonstrated that the geotropic curvature 

 is an active and not a passive one ; that both it and the 

 converse apogeotropic one depend on unequal distribution 

 of longitudinal growth ; and that they are not connected 

 at all with passive variations of tensions of tissues in the 

 growing organs. In the second memoir, Die natiirliclie 

 wager echte Richtung der Pftanzentheilen, he showed that 

 horizontal growth proceeds on the same lines as vertical 

 growth, and that it also is influenced by gravity. He 

 proved geotropism to be a similar phenomenon to helio- 

 tropism, and set out for the first time definitely the theory 

 that growing organs react to the influences of gravity 

 and of unilateral light by variations in the details of their 

 growth. He showed further that different members or 

 organs respond differently to a uniform force, and that 



