hij the aid of Polarized Light. 271 



When, for example, a spiral vessel with a gently-ascending spiral 

 is inclined at an angle of 45° to one of the Nicols, and thus the 

 fibres of its posterior and anterior sides react in the same way 

 on polarized light, in the manner of transverse fibres, appearing 

 of a yellow colour, the places of curvature of the same will be 

 blue, thus appealing of the same colour as if the lateral walls 

 were composed of longitudinal fibres. The places of curvature 

 appear of the same colours if the fibres of the anterior and 

 posterior sides ascend at an angle of 45° to the longitudinal 

 axis of the vessel, and in consequence of this, in the said posi- 

 tion of the vessel remain uncoloured. When, on the other hand, 

 the fibres describe a very steeply-ascending si)iral, and the fibres 

 lying on the posterior and anterior sides of the vessel thus 

 act in the manner of longitudinal fibres, their colour agrees 

 with that of the places of curvature. Constant and regular 

 as these phtcnomcna appear when wc examine organs of very 

 regular structure, like spiral vessels, in examining thick-walled 

 cylindrical cells, for instance the hairs of Buraginca, or isolated 

 prismatic cells, such as wood- and liber-cells, many exceptions 

 will be found, partly because the latter are subject to much 

 irregularity in the course of the fibres from the oblique, variously 

 changing inclinations of the side-walls of the cells, partly because, 

 as it appears, the direction of the fibres is not always the same 

 in the different layers of one and the same cell, so that, for in- 

 stance, it describes a less steep spiral in the outer lamellae of the 

 cell-wall than in the inner. 



The phrenomena which the cell-walls exhibit in polai'ized 

 light undergo, again, manifold modifications when two cell- 

 membranes with differently directed fibres lie immediately one 

 over the other. This naturally occurs with extreme frequency 

 in the cellular tissue of plants when the fibrillation of the cells 

 follows a spiral direction, since in that case the fibres will pursue 

 opposite directions on the coherent membranes of two adjacent 

 cells. When two such membranes are viewed lying one over 

 the other in a horizontal direction, the lower one, as a doubly- 

 refractive substance, must exert upon the light coming from the 

 Nicol an analogous although weaker influence to that of a plate 

 of selenite, and the effect of this must be more or less clearly 

 indicated in the pha^nomena presented by the upper membrane. 

 An influence of this kind must likewise make itself felt even 

 when the membranes do not lie immediately one on the other, 

 but are separated by a greater or less space, as for example the 

 upper and lower membranes of a cell by the cell-cavity. Here, 

 again, the spiral vessels of Musa may be used as a suitable ob- 

 ject, in which these phscnomena may be observed with certainty. 



If we examine a vessel, placed perpendicular to one of the 



