174 PLANT PHYSIOLOGY 



Cell-division may take place even where there is not enough 

 room for growth, but it does not constitute an essential 

 part of the process of growth, though it usually precedes 

 growth and makes it possible. An organ consisting of two 

 hundred cells has not grown in any sense when these cells 

 by mere division have become four hundred. Under normal 

 conditions, however, growth will follow, increase in volume 

 and in amount of protoplasm taking place when there is 

 enough room.* 



Growing parts exert, or may exert, great mechanical 

 force. Illustrations of the truth of this assertion may be 

 observed almost daily. \ Until recently, however, the at- 

 tempts to determine the amount of force which a growing 

 part can exert have yielded only inadequate results. It is 

 necessary to employ such apparatus that all the force de- 

 veloped by the growing plant will be exerted directly upon 

 the recording instrument. The best instrument so far de- 

 vised is Pfeffer's.]; 



Pfeffer's description of his apparatus will explain the ac- 

 companying illustration, reduced from the original figure. 

 "The spring is supported on an iron bar (d) 14 mm. thick 

 which is rigidly attached by means of double screw clamps 

 (e) to the upright posts (ss) of the stand. The measuring 

 spring (/) can be changed, for the plate (7) which carries it 

 is fastened by the screws (kk) to the solid brass plate (#). 

 By raising or lowering this plate the spring can be moved 

 up or down, to or from the plaster-of-Paris block (a). 

 For this purpose the plate (#) rests upon three screws (7;) 

 which pass through correspondingly threaded holes in the 

 flattened and expanded part of the bar (d). Upon the 

 metal plate on the upper side of the spring is fastened the 



* On the effects of mechanical restraint on the growth and other behavior 

 of plant-cells and parts, consult Newcombe, F. C. Influence of mechanical 

 resistance on the development and life-period of cells. Botanical Gazette, 

 vol. 19, 1894. Pfeffer, W. Druck und Arbeitsleistung durch wachsende 

 Pflanzen. Abhandlungen d. K. Sachs. Gesellsch. f. Wissensch., Bd. XX., 1893. i 

 Also p. 187 of this book. 



fFor striking observations under this topic see Kerner and Oliver's 

 Natural History of Plants. Vol. I, part 2. pp. 513-17. 



J Pfeffer, W. Druck und Arbeitsleistung. 



