[PENU allow] a birch rope 243 



to be sufficient to rupture this layer, so that upon sectioning, or even 

 upon inspection of the original specimen, the layers appear quite dis- 

 tinct and separate from one another. A comparison of the layers 

 throughout the entire transverse section shows considerable variation in 

 thickness of such a nature that there is a more or less graduated diminu- 

 tion from within outwardly. Thus the outermost perfect layer of cork 

 has a thickness of 17.8 1.1., while the innermost layer has a thiclmess of 

 53.4 yu, giving a ratio of 1 : 3. From this it is evident that the 

 various layers must have been subjected to compression in growth, and 

 an attempt was therefore made to obtain a confirmation or denial of 

 this result from another point of view. Accordingly measurements were 

 taken of the radial diameter of the cork cells in the inner and outer 

 regions of growth, as well as on the inner and outer facesi of the indi- 

 vidual layer in the two situations, with the following results: — 



innermost laver of cork united to parenchyma. 



Inner face 8.70 yw 



Outer face 7.00 yu 



Second or first completed layer of cork. 



Inner face 8.70 pi 



Outer face 5.26 // 



One 01 the outermost layers of cork, immediately be- 

 neath the bleached layers. 



Inner face 3.51 yw 



Outer face 3.51 /< 



These results taken collectively, show most conclusively, that the 

 layers of cork have been developed under considerable tension resulting 

 from the resistance of the outermost and first formed layers. But the 

 second set of measurements also shows very convincingly, what might 

 have been anticipated on theoretical grounds, that while there is an 

 augmenting tension toward the outermost layers, the same rule applies 

 to the individual layer in such way that the outermost cells are always 

 of inferior radial diameter. The general result is expressed in the meas- 

 urements which show that the cells of the outermost layers are to those 

 of the innermost layers, where the conditions were most favourable to 

 free development, in the ratio of 1 : 2.48, a fact which is also apparent 

 from an inspection of figures 2 and 7. 



Some interesting structural variations have been noted, and these 

 may be briefly considered. Typically, the central region of parenchyma 

 is surrounded, terminally as well as laterally, by well developed cork in 

 successive layers. The parenchyma itself is somewhat more dense at 

 the extremities where it does not readily admit of fracture at maturity 



