PROGRESS IN PLANT PHYSIOLOGY 83 



flow. He traced the pathway of ascent of sap by Hales's 

 method of ringing, and by immersion of the cut ends of 

 branches in an infusion of grape skins. He thought 

 that " perspiration " (transpiration) was effected directly 

 by the under surface of the leaf while the upper surface 

 absorbed water. Most of the water came from a reservoir 

 in the pith, so that his knowledge of root absorption 

 cannot be regarded as very accurate or profound. The 

 old notion of peristalsis in the wood vessels as a cause of 

 the ascent of sap reappears in Knight's papers, for he 

 suggests that the movement may be due to rhythmic 

 expansion and contraction of the " silver grain " under 

 the influence of heat. 



In the discussion of these and similar problems in 

 nutrition Knight is far behind Hales, who lived almost 

 a century before his day. But in one section of physiology, 

 sensitivity, he made experiments that are still quoted, and 

 drew deductions from them that are incorporated in all 

 our present-day accounts of the phenomena of stimulus 

 and response. At the same time you must not imagine 

 that Knight had any conception of plant sensitivity 

 as we understand the term. Geotropic and hehotropic 

 curvatures were to him simple bending movements that 

 had to be explained mechanically ; protoplasm with all 

 its mysterious capabilities for appreciating and responding 

 to stimuli was to Knight a sealed book. 



In a paper read to the Royal Society in 1806 he opens 

 the question by stating that while studying seed germina- 

 tion he had observed that no matter how he placed the 

 seed " its radicle invariably makes an effort to descend 

 towards the centre of the earth, while the elongated 

 germen (plumule) takes a precisely opposite direction." 

 He then hints that " some naturalists have supposed 

 these opposite effects to be produced by gravitation." 

 Knight, however, was not content to let the matter rest 

 there ; he set himself the task of finding out whether 

 gravitation had or had not anything to do with the 



