PHYSICAL PROPERTIES OF CORK 139 



ordinary air, even under considerable pressure. Wiesner's apparatus 

 consists essentially of a glass T-tnbe. The plate of cork which is to be 

 tested is cemented over the end of one transverse arm ; mercury is 

 sucked into the vertical arm through a rubber tube (provided with a 

 screw-clamp), which is attached to the other transverse arm. Small 

 plates prepared from the corky tissue of Qitercus Suber, not exceeding 

 05 to "07 mm. in thickness, and hence comprising only two or three 

 layers of cells, were in this way exposed to a (negative) pressure of 

 20 cm. mercury for several weeks without any leakage of the outer air 

 taking place. With a suitable arrangement of the apparatus a plate of 

 cork consisting only of three layers was found to be quite impervious 

 to air, even under a difference of pressure amounting to a whole 

 atmosphere. 



Schwendener S3 has shown that corky tissues are in general both 

 inextensible and inelastic. Isolated strips of the periderm of Castanea 

 vesca, Fagus sylmtica and Cytisus Laburnum break before their longi- 

 tudinal extension has reached 2 per cent. The thin-walled cork-layers 

 of the Beech, which consist mainly of suberin, display a similar 

 behaviour ; the thick- walled layers on the contrary, in which cellulose 

 predominates, are very extensible. The author has found that 

 tangentially stretched strips of the periderm of arm-thick branches of 

 Tilia grandifolia break when their elongation amounts to 4 per cent., 

 whereas similar strips taken from two-year-old twigs can be stretched 

 to the extent of 7 per cent, or 8 per cent. ; the periderm thus seems 

 to be somewhat more extensible while it is young. According to 

 Schwendener the peridermal covering of Primus is exceptional in this 

 respect since it can undergo an extension of 10-12 per cent, without 

 breaking. An apparent exception has been noted by the author in the 

 case of ordinary bottle-cork ; strips of this material cut in a tangential 

 plane can be stretched to the extent of 25 per cent, and remain 

 perfectly elastic up to elongations of 6-7 per cent. It is in all 

 probability this remarkable stretching power of bottle-cork which is 

 responsible for the old notion that corky tissues in general are highly 

 extensible ; the peculiar behaviour of bottle-cork, however, depends 

 upon the arrangement of the component cells and not upon the 

 physical properties of their walls. In the case of bottle-cork, namely, 

 the cells of adjacent radial series alternate with one another ; hence 

 when a strip of cork is stretched in a tangential direction, the radial 

 cellr walls are thrown into zig-zag folds. The elongation produced in 

 this way is large simply because the radial diameter of each cork-cell 

 is considerable. This statement can be easily verified by observing a 

 strip of cork under the microscope while it is being stretched ; it is 

 further borne out by the fact that radial strips cannot be stretched 



