268 H. von Mohl on the Investigation of Vegetable Tissue 



the fibres approximate to the position of transverse fibres, in the 

 above-mentioned position of the vessel, the fibres of the anterior 

 and posterior walls act in the same way upon polarized light, 

 and the vessel appears of the same colour behind and in front, like 

 a cell clothed with cross-fibres, or like a section of a cell-wall 

 passing at right angles to the long axis of the vessel. When 

 the spiral vessel is so far drawn out that the fibres of its anterior 

 and posterior walls cross at right angles, in the oblique position 

 of the vessel above mentioned they will stand perpendicular to 

 the two Nicols, and consequently will not act upon polarized light. 

 When, lastly, the vessel is so much drawn out that the fibres 

 form a very acute angle with the longitudinal axis of the vessel, 

 they act like longitudinal fibres, and hence appear with the 

 complementary colours of those tints which are exhibited by the 

 fibres of a closely- wound vessel. 



It need scarcely be specially mentioned that the behaviour 

 of spiral vessels is shared by cells which contain spiral fibres, for 

 instance the elaters of the Liverworts, the leaf-cells of Sphag- 

 num, the cells of the sporange of Equisetum Telmateia, the elaters 

 of the same plant, the spiral cells in the stem and petiole of 

 Nepenthes, those of the Orchidese, &c. 



Completely analogous conditions are observed in those ele- 

 mentary organs whose secondary fibres form a more or less 

 regular network, as in the reticulated vessels of the Monocoty- 

 ledons, the scalariform ducts of the Ferns, the reticulated paren- 

 chyma-cells of the wing of the seed of Swietenia Mahogani, &c. 

 Of course the fibres here do not agree in colour in any position 

 of the cell, but exhibit, according to their varied positions, the 

 same colours as the cell-walls in the section of a parenchymatous 

 cellular tissue. 



As is well known, we find transitions from those cells which 

 have the secondary layers divided into separate fibres, to the 

 apparently homogeneous cell-membranes, or structures of that 

 kind where the secondary layers are not composed of separable 

 fibres, yet possess a fibrous structure, indicated by a finer or 

 coarser striation, and are more easily torn in the direction of 

 these streaks than in any other direction. This condition gave 

 rise to the notorious dispute whether or not cell-membrane is 

 composed of primitive fibres. 



Among unpitted cells with fibre-like striation of the mem- 

 brane, many Confervse, for example C Melagonium, and further, 

 the Cladophorce, are well known to be remarkable for the cir- 

 cumstance that they exhibit two systems of such streaks, crossing 

 at right angles. It was a question here whether one alone of 

 these systems determined the optical character of the membrane, 

 or the two exerted an equal and opposite effect, mutually aeu- 



