by the aid uf Pularizcd Liyhl. 2G9 



tralizing their action, as occurs in two cross plates of mica. 

 Observation shows that the former is the case, and that the 

 colour which the membrane exhibits on the appHcation of a plate 

 of selenitc depends on the direction of the longitudinal striation. 

 The direction of this striation is not the same in all Cladophorae. 

 In some, as in C. ylomfrata and lonyissima, one system of striae, 

 determining the optical conditions, runs parallel with the long 

 axis of the cell ; hence the cell-membrane of these species ap- 

 pears colourless when the cell is placed perpendicular to the 

 Nicols, and in an oblique position exhibits the same colours as 

 a section through a cell-wall lying parallel with the cell. In 

 other species, as in C. hospita, the longitudinal stria? run in the 

 direction of a steeply-ascending spiral, and the action of the light 

 is altered accordingly. 



In an analogous way, the cell-membranes of the Characese 

 also exhibit two systems of streaks, crossing at right angles. 

 But the striation is difFei'ent from that of the Cladophoroe. It 

 does not present, as in them, uniform and parallel lines, but the 

 cells appear traversed in a transverse direction by fine streaks, 

 which form a network with long meshes, while thicker bundles 

 of fibres run longitudinally, often exhibiting an undulating 

 course. This condition occurs both in the central cell of Chara, 

 for example in C. equisetina, Kiitz., and in the cell-wall of 

 Nitella, for instance in N. flexilis, mucronata, fasciculata, and 

 syncarpa. These membi*anes act very powerfully upon polarized 

 light, but, what is remarkable, they exhibit in the same positions 

 the opposite colonics to those of the Cladophorce. This might 

 give rise to the conjecture, that the Charat form, in regard to the 

 optical conditions of their cell-membranes, similar exceptions to 

 the general rule to those in Caulerpa ; but the examination of 

 transverse sections of Nitella fasciculata and Chara equisetina did 

 not confirm this, for these behaved exactly like the transverse sec- 

 tion of an ordinary cell. Since, then, on the lateral view of the 

 cell the colouring is the opposite of that of the cell-membrane 

 of Cladophora, and agrees with that of a spiral vessel with a very 

 gently-ascending fibre, it is clear that in the Char(E the optical 

 character of the membrane is determined by the transverse 

 fibres. 



Phaenomena exactly analogous to those of cells in which the 

 membrane has visible fibre- like streaks, are exhibited by cells in 

 which only traces of this a])pearance are to be detected, or whose 

 membrane appears to be completely homogeneous ; since their 

 membrane, according to the direction in relation to the Nicol, is 

 sometimes invisible, sometimes more or less brightly illuminated, 

 and, on the application of the selenite-plate, developes a yellow 

 or blue colour. Here exists, on the one hand, evidence that the 



