Autonomy of Vacuoles 151 



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This was still more the case through the demonstra- 

 tion furnished by the same author, that the tension to 

 which growing cell-membranes are subjected by the cell- 

 sap is one of the most essential mechanical causes of the 

 surface growth of these membranes. For with this dem- 

 onstration Sachs laid the foundation still valid, for the 

 whole mechanical theory of growth in length. 



Building on this foundation, many investigators have 

 enlarged our knowledge of the mechanical causes of 

 growth in various directions. Some have especially 

 measured and analyzed the degree of extensibility of the 

 cell-membranes and the amount of force supplied by the 

 cell-sap. Others have studied the causes governing the 

 variations of extensibility of the wall in one and the same 

 cell, and which occur in different spots and in different 

 directions, and have explained them, as due, with great 

 probability, to local differentiations in the protoplast it- 

 self, which might regulate this elasticity through the 

 secretion of certain enzymes. Others again have at- 

 tacked the doctrine of intussusception, which was the 

 prevailing one at the time of the discoveries mentioned, 

 have proven it to be incorrect, and have tried to ressusci- 

 tate in its place, in a new form, the old "apposition 

 theory." 



Although subject to misunderstandings from some 

 sides, 50 Sach's theory has acquired a prominent position 

 in plant-physiology, and, since the two decades of its es- 

 tablishment, it has become, in ever increasing measure, 



50 In my "Untersuchungen iiber die Mechanischen Ursachen der 

 Zellstreckung" (p. 3, 1877.), I have distinctly emphasized the fact 

 that there are also phenomena of growth independent of turgor, 

 and that therefore this turgor is neither the only, nor even the first 

 reason for growth. Krabbe and Klebs arrived later at the same 

 conclusion. Cf. Arbeiten Bot. Inst. Tubingen. 2: 530. 1888. 



