270 H. von Mohl on the Investigation of Vegetable Tissue 



apparently homogeneous cell-membrane is not really so ; and on 

 the other hand, the position in which the cell must be placed in 

 order to be visible, and the colour produced by the selenite- 

 plate, enable us to ascertain the direction of its invisible fibril- 

 lation. In this respect, for example, isolated vessels from the 

 trunks of Tree-ferns are very instructive : on their membranes 

 occur largish spots, smooth and perfectly homogeneous, corre- 

 sponding to the angles of adjacent cells; on the application of 

 the selenite-plate, these exhibit the same colour as the fibres 

 running between the pits, and hence demonstrate that the entire 

 membrane has a fibrous structure in the transverse direction. 

 Beautiful examples of this are furnished also by the wood- 

 cells of the Coniferse and Cycadese, in which, very frequently, 

 even when a definite fibrillation is not indicated by striation of 

 the membrane with the ordinary microscope, the direction in 

 which the fibres* run may be determined by the angle at 

 which the slits of the pits stand obliquely to the long axis. 

 This angle amounts, for instance in the wood- cells of a Cycas, 

 to nearly 45°. If the cells are placed perpendicularly to one 

 Nicol, these membranes appear, according as the fibres are 

 directed to the right or left, either blue or yellow ; on the other 

 hand, they become colourless when the cell is inclined at an 

 angle of 45° to the Nicol, while in this position the illumination 

 and brilliancy of colour of the side-walls standing perpendicular 

 to the surface of the object-slider attain their maximum. An 

 analogous behaviour is exhibited by the cells of Torreya taxi- 

 folia, whose fibres ascend at an angle of 70°, and the cells of 

 Fir-wood, with an angle of 68°, — only these, for reasons readily 

 explained, did not show the greatest brilliancy of colour when 

 the cell was placed perpendicular to a Nicol, but when it was 

 so far inclined that its fibres formed an angle of 45° with the 

 Nicol. 



Peculiar phsenomena are exhibited by the lateral borders of a 

 cell containing an oblique or transverse fibre on its walls. The 

 longitudinal section of a cell-wall behaves to polarized light, as 

 above noticed, like a transverse section of the same; and the 

 same holds good, in spiral vessels, annular vessels, scalariform 

 ducts, &c., of the side borders at which the fibres of the anterior 

 side curve downwards to reach the posterior side. In con- 

 sequence of this, the fibres present, at the places of curvature, 

 sometimes the same, sometimes the opposite colour from that of 

 the fibres running on the horizontally-lying lateral surfaces, 

 according as the latter ascend in a more or less steep spiral. 



* When I speak here and in other places of fibres, I do it for the sake 

 of brevity of expression, and by no means thereby intend to defend the 

 doctrine of the existence of primitive fibres. 



