CHAPTER I. THE CELL. 



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3. The formation of lignin, or wood-substance. This con- 

 stitutes the great bulk of the wood of trees, and differs in chem- 

 ical properties both from cutin and cellulose. The middle 

 lamella, already described, is a form of lignin. 



4. The infiltration of coloring matter. This not infrequently 

 occurs in old and lignified cell-walls, such as those of the heart- 

 wood of trees. The colors thus produced are often quite charac- 

 teristic, as, for example, those of the wood of 

 Cherry, Mahogany, Walnut, and Red Saunders. 



5. The infiltration of mineral matters. The 

 commonest of these are silica and calcium salts. 

 Beautiful examples of the former occur in the 

 cell-walls of Diatoms, where the silicification is 

 very complete, and the silicified walls are often 

 very delicately sculptured. Notable amounts of 

 silica also exist in the walls of many plant- 

 hairs, and in the ordinary epidermal cells of 

 the Equisetums and many of the Grasses. 



Calcium carbonate also frequently occurs in 

 the cell-walls of hairs, in those of some sea- 

 weeds, as the Corallina, Acetabularia, etc., and 

 in the curious cell-wall modifications called 

 cystoliths found in the leaves of plants belong- 

 ing to the Nettle and Acanthus families. See 

 Fig. 389. Calcium oxalate occurs occasionally 

 as a crystalline deposit in the walls of thick- 

 walled cells, as in those of Welwitschia, see 

 Fig- 367, but more commonly crystals of this 

 kind are found among the cell-contents. 



6. The conversion into mucilage, or gum. 

 Examples of this kind of change occur in the 



outer cells of the seeds of the Quince, Flax, many Polemoni- 

 ums, etc. If any of these seeds be placed in water, the walls of 

 the outer cells may be observed to swell, become transparent, 

 and finally dissolve, forming a thick mucilage. The cell-walls 

 of the pith and medullary rays of some plants become changed 

 into gum. This is the case with the Astragalus plants that 

 yield gum tragacanth. 



Fig. 367. -Half of a 

 thick-walled cell from 

 Welwitschia mirabilis, 

 showing crystals of 

 oxalate of calcium im- 

 bedded in the cell- 

 wall. After Sachs. 



