CELLULOSE. 31 



which cannot be satisfactorily explained by this hypothesis. 

 There are, besides, numerous instances in which the stratified 

 appearance is not clearly shown until the cell has been acted 

 on by an acid or an alkali ; a good example of this is afforded 

 by the firm cells of the albumen of the vegetable ivory (Phy- 

 tclephas) .* 



139. An appearance of spiral striation, 2 ascribed also to the 

 unequal distribution of water, is often seen, especially in the 

 cells of the liber of Apocynaceae and allied orders, and in many 

 wood-cells. The striations are not constant as regards the 

 steepness of the spiral ; in fact, in a few instances rings instead 

 of spirals are present. A striated appearance is sometimes pre- 

 sented in walls which have been deprived of all their water. 



140. Chemically considered, the vouno; cell-wall consists essen- 



j , <_? 



tially of cellulose, a substance which has the same percentage 

 composition as starch, namely, C II 10 O 5 . Even in its purest 

 state it is associated with a trace of mineral matters which 

 remain behind as ash when it is burned, and in the living cell it 

 is always permeated by water. 



*/ */ 



141. Cellulose is not soluble in an}' of the following liquids 

 commonly used in microscopic manipulations, - - water, alcohol, 

 glycerin, dilute alkalies, and dilute acids. It is. however, more 

 or less strongly acted on by hot concentrated alkalies, without 

 passing into true solution, and it is apparently dissolved by 

 strong sulphuric acid. Whether cellulose becomes truly dis- 

 solved by concentrated sulphuric acid, or merely forms some 

 other carbohydrate under its action, is of little consequence, so 

 far as the destruction of cell-walls is concerned. In nearly all 



V 



cases its action is so energetic that the wall of a cell can be 



1 As shown by Mohl, the action of a mineral acid of proper degree of con- 

 centration causes the wall to swell up, and the lamellar structure becomes 

 very distinct. " By this means the lamellar structure maybe demonstrated 

 even in those cases in which the unaltered membrane appeared completely 

 homogeneous" (Mohl: Vegetable Cell, p. 10). 



2 "The stratification invisible on the transverse and longitudinal sections 

 of the cell-wall, the striation on the surface as well ; it is usually most evident 

 there, but is in general less easily seen than the stratification ; it depends on 

 the presence of alternately more or less dense layers in the cell- wall, meeting 

 its surface at an angle. Generally two such systems of layers may be recog- 

 nized mutually intersecting one another. There are thus all together three 

 systems of layers present in cell-wall : one concentric with the surface, and two 

 vertical or oblique to it mutually intersecting, like the cleavage planes of a 

 crystal splitting in three directions (Na'geli) ; and just as this cleavage is some- 

 times more evident in one direction, sometimes in another, so it is also with 

 the stratification and striation " (Sachs: Text-book, 2d Eng. ed., p. 20). 



