CELLULAR TISSUE. 37 



cially of the poorer sorts. A section of a few cells of the kind is 

 represented in Fig. 16, with their 

 cavity much reduced and rendered 

 very irregular in outline by such in- 

 crustation. Similar cells are readily 

 seen, with a moderate magnifying 

 power, to form a part of the tissue 

 even of such juicy fruits as the cranberry and the blueberry 

 (Fig. 17). 



40. This deposited matter can rarely consist of pure cellulose, 

 but may include all the various matters capable of solidification, of 

 whatever nature, which are introduced into the cells from without, 

 or are elaborated there. As, for example, mineral matters, small 

 quantities of which must needs be dissolved in the water which 

 the plant imbibes by its roots, and be deposited in the cells of 

 the wood through which it passes, and especially in those of the 

 leaves, where it is concentrated by evaporation, as will be here- 

 after illustrated ; also, coloring matters, such as give the different 

 tints to heart-wood, and other special solidified products formed 

 in the cells themselves. The cells fill up, therefore, partly by or- 

 ganic deposition, and partly by incrustation. 



41. Even when purified as much as possible from all admix- 

 ture of foreign matei^als, the secondary deposit is found to differ 

 a little from cellulose, or original cell-membrane, in chemical 

 composition. It contains a somewhat larger proportion of carbon 

 and hydrogen, and is therefore richer in combustible matter. 

 Forming as it does the principal part of the weight of wood (lig- 

 num), it has received the name of LIGNINE (also that of Sclero- 

 gen) ; but it is probably only cellulose a little modified or altered. 

 This difference in chemical composition, however, shows why the 

 hard woods, such as hickory and oak-wood, which abound in this 

 lignified deposit, should be more valuable for fuel, weight for 

 weight, than the soft woods, which have little of it (such as bass- 

 wood, &c.) ; at least, when the latter are not charged with resinous 

 matter. 



42. The secondary deposit often forms an even and continuous 

 increase in the thickness of the walls (as is shown in the White 

 Oak, in the section on the internal structure of the stem) : but it is 



FIG. 16. Magnified section of the gritty cells of the pear ; the cavity almost filled with in- 

 crusting matter. 17. Similar cells from the pulp of the blueberry (Vaccinium corymbosum). 



4 



