162 



MECHANICAL SYSTEM 



The experimental method employed by Schwendener is quite 

 straightforward. Strips of tissue, 150-400 mm. in length and 

 2-5 mm. in breadth, are prepared from leaves, or from stems 

 containing a large proportion of bast; one end of each strip is 



firmly fixed in the jaws of 



JQOT 



oc 



a vice, while to the other 

 extremity a clamp is fastened 

 to which weights can be at- 

 tached. Simple accessory con- 

 trivances serve to measure the 

 extension produced by a given 

 load, and the subsequent con- 

 traction which takes place 

 when the weight is removed. 

 From each strip which has 

 been tested in this way 

 transverse sections are pre- 

 pared : these are drawn to 

 scale under suitable magnifica- 

 tion with the help of a camera 

 lucida, and the cross-sectional 

 area of the resistant elements 

 is calculated from this drawing. 

 Am bron n employed a similar 

 method to determine the 

 tenacity and elasticity of 

 collenchyma : he took special 

 precautions with a view to 

 estimating the mechanically 

 effective cross-sectional area as accurately as possible. The trans- 

 verse section of each collenchymatous strand was drawn upon 

 paper of uniform thickness with the aid of a camera lucida : the 

 drawing was then cut out and weighed. Xext the areas corresponding 

 to the cell-cavities were carefully removed ; from the weight of the 

 remaining paper net-work the cross-sectional area of the walls could 

 then be readily calculated. 



The subjoined table comprises a series of data selected from the 

 measurements of Schwendener, Weinzierl, and Firtsch, to which are 

 appended, for the sake of comparison, the corresponding values for 

 certain metals : some of the latter (which are quoted from the fifth 

 edition of Weisbach's Ingenieur-und Maschinen mechanik) are included 

 in Sehwendener's treatise on the " mechanical principle." 



Fig. 54. 



Part of a T.S. through the leaf of Camellia 

 japonica, showing one of the multiradiate scle- 

 reides. 



