STRIATION OF THE CELL-WALL. 



91 



Besides these concentric layers there is to be recognised, however, in very many cases 

 in thick cell-walls, occasionally also in thinner ones, the so-called striation, which is 

 perceived best With very strong magnifying powers when the cell-walls are seen in 

 surface-view. The striation appears then in the form of parallel lines, which 

 generally run obliquely across the object, or travel round the cell-wall in the form 

 of a spiral, or even in closed circles. In the first case are recognised occasionally, 



Fig. 91. — Forms of cells at Marchaniia polymorpha (a 

 Liverwort)- with thickenings prBjecting-inwards. A half an 

 elater from the sporogonium, with two spiral bands; A* a. 

 portion more highly magnified; B a parenchyma cell ft-oln 

 the middle portion of the thallus, with reticulate thickenings 

 projecting inwards ; C a fine root-hair with thickenings pro- 

 jecting inwards^-these are arranged on a spiral constric* 

 tion of the cell'wall ; D a thicker root-hair'^the projections 

 are thicker and branched, and the spiral arrangement still 

 more evident. 



SiiHter^ 



Fig. 92.— ^-Portion of an annular vessel from 

 the fibro-vascular bundle of Zea Mays (X 550). 

 k, h the thin cell-watl of the vessel, on which 

 are clearly to be seen the boundary lines of the 

 neighbouring cells ; r, r the annulai thickenings 

 of the wall of the vessel ; y the inner substance 

 of one of the rings cut through transversely ; 

 i the deuser layer which extends over the ring 

 on its inner side projecting into the lumen of 

 the cell 



with sufficient magnifying power, two systems of striation crossing one another. 

 Nageli has also referred the striations of the cell-wall to the existence of layers, 

 alternately richer and poorer in water, which traverse the cell-wall from without 

 inwards. The whole cell-wall, according to this view, resembles a crystal 

 cleavable in three directions, the intersecting lamellae of which are alternately 

 richer and poorer in water. 



In addition to this very fine structure of the cell-wall, we have to consider, 



