THE CELL-WALL. 



29 



The corresponding growth in thickness is less marked if the thickenings show a 

 distinctly spiral structure, or if they arise in the form of strong spiral bands, as in spiral 

 fibre-cells ; if, in this case, in each pair of adjoining cells one or more spiral bands wind 

 in the same direction, they must necessarily cross on the common partition- wall. 



(c) Stratification and Striation of the Cell-wall \ When the cell-walls have at- 

 tained a certain thickness and extent of surface, stratification and striation take place 

 more or less clearly. In consequence of stratification 

 the cell-wall appears composed of very thin membranes 

 enclosed one within another and fitting very closely 

 together; the stratification is seen both on the transverse 

 and the longitudinal section of the cell-wall. The stria- 

 tion is generally to be seen most plainly from the front ; 

 it may be observed in the form of two systems of lines 

 (sometimes apparently several) which are marked on 

 the upper surface. The one system, consisting of par- 

 allel stria!, is always cut by the other system, which 

 also consists of parallel striae. A closer investigation 

 shows that the structure which gives the appearance 

 of striation does not belong merely to the upper 

 surface or to one layer of the cell-wall ; but that the 

 striation rather penetrates the whole thickness of the 

 cell-wall, and that the striae are thus lamellae which 

 cut the upper surface, and are continued through all 

 the concentric layers. If the striation is very clearly 

 marked, and if it is nearly parallel to the longer axis of 

 the cell, it may also be recognised under transverse sec- 

 tion in the form of striae, which penetrate the concentric 

 layers; when the cell-wall is cut lengthwise only those 

 systems of striation are to be clearly recognised which, 

 seen from the surface, run almost diagonally round the 

 cell. 



Every system of stratification or striation con- 

 sists of lamellae of visible thickness and of different 

 refractive powers, so that a more strongly refractive 

 layer or stria always alternates with a less strongly re- 

 fractive one. This diff'erence of refraction results from 

 a diflferent distribution of water and of the solid 

 particles in the cell-wall; the less strongly refractive 

 lamellae are more watery, poorer in cellulose, and thus 

 less dense ; the more strongly refractive and denser 

 lamella! contain less water and more cellulose. Hence 



stratification and striation of the cell-wall disappear when water is completely elimi- 

 nated, as also when it swells violently, /. e. absorbs much water ; because, in the first 

 case, the more watery layers are reduced to the condition of the less watery ones, in 

 the latter case the less watery, by plentiful absorption, become similar to the others. 

 On the other hand stratification and striation become most conspicuous when for a 

 given proportion of w^ater in the cell-wall the difference between the dense and the soft 

 layers is greatest. In many cases this may be brought about by addition of acids or 

 alkalies which occasion a moderate swelling. But if the dense layers are very dense, 



Fig. 32.— Bast-cells from the leaf of Hoya 

 carnosa [x^oo), showing the striation. These 

 are not nearly so strongly marked in nature, 

 but are quite as plain ; a optical longitudinal 

 section of the crossed annular striation ; 

 b external view of the side where the an- 

 nular striation cross ; c external view of the 

 side where they do not cross ; d the same ; 

 e a piece of cell-wall, where only single 

 annular striations can be seen. 



1 H. von Mohl, Bot. Zeitg. 1858, pp. 1,9.— N.-lgeli, Ueber den inneren Bau der vegetabilischen 

 Zellenmembran, in the Sitzungsbekchte der Miinchener Akad. der Wissenschaften, 1864, May and 

 July. — Ilofnieister, Lehre von der Pflanzenzelle, p. 197. 



