36 THE ELEMENTAEY STRUCTURE OF PLANTS. 



wood, ami especially in those of the leave--, where much of the water 

 escapes by evaporation, — and sometimes certain coloring matters 

 also, such as give the different tints to heart-wood, &c. Even 

 when purified as much as possible from all admixture of foreign 

 materials, the secondary deposit is said to differ a little from cellu- 

 lose, or original cell-membrane, in containing a somewhat larger 

 proportion of carbon and hydrogen : it is therefore richer in combus- 

 tible matter. Forming as it does the principal part of the weight 

 of wood (lignum), it has received the name of Lignine (also that of 

 Sclerogen) ; but it is only cellulose a little modified. This differ- 

 ence in chemical composition, however, shows why the hard woods, 

 such as hickory and oak, which abound in this lignified deposit, 

 should be more valuable for fuel, weight for weight, than the soft 

 woods, which have little of it ; at least, when the latter are not 

 charged with resinous matter.* 



43. The section of the wall of a cell thickened by internal 

 deposit, when moderately magnified, commonly appears to be homo- 

 geneous and uniform. But under a high magnifying power it may 

 often be distinguished more or less distinctly into successive con- 

 centric layers (Fig. 27-31). However this may be, it rarely hap- 

 pens that the thickening deposit is spread evenly over the whole 

 inner surface of a cell. It is commonly interrupted or much thinner 

 at some places, so as to give the diminished cavity of the cell very 

 irregular outlines (as in Fig. 27, 28) ; or else it is wanting at cer- 

 tain small and definite spots, which, being more transparent, Avhen 

 looked down upon from the outside appear like holes or pores (Fig. 

 32, 66, 57) or slits (Fig. 58, 59), according to their shape. In this 

 way are formed the various 



44. Markings of the Walls of Cells. These, whether in the form of 

 bands, spiral lines, dots, or appai-ent pores, all arise from the unequal 



* From the manner in which the thickening takes place, it would appear that 

 the innermost layers must always be tlie most recent. But M. Tre'cul has con- 

 vinced himself that the primary cell-membrane sometimes produces a secondary 

 one outside of itself, as well as on the inside, so that the original cell-wall is 

 intermediate. And also, that, when the thickening deposit is wholly within the 

 primary wall, the intermediate layers are occasionally secreted in some way by 

 the outer or inner ones, and therefore more recent than the inner. Unlikely 

 as all this seems, M. Trecul's investigations are entitled to great attention. 

 His elaborate memoir, upon Secondary Formations in Cells, is published in the 

 Annales des Sciences Naturelles, 4th scr. Vol. II. 1854. 



