THE CELL-WALL. 



29 



plasm in one cell with that in contiguous cells will be considered 

 in Chapter VI. 



128. The cell-wail. The cell-wall is produced from materials 

 contained in protoplasm, ' and is laid down in intimate contact 

 with it, as an even homogeneous film which exhibits at first no 

 obvious structure, but with increase in size generally' becomes 

 modified in appearance, consistence, and composition. 



129. Its evenness of surface is in most cases early lost by 

 addition of new matter, giving rise to protuberances or marliings 

 of diflTerent sorts. Though at first possessing no evident struc- 

 ture, it may become clearlj' difll'erentiated into laj-ers, and thus 

 become stratified, or striations maj- ajipear. Its consistence, at 

 the outset that of the most delicate bleached linen fibre, maj' 

 soon become changed, on the one hand to that of soft gelatin, 

 or on the other to that of the densest wood. Moreover, although 

 devoid of color when first produced, it maj' acquire distinct color- 

 ation ; and, lastlj-, its chemical character nia3' undergo such im- 

 portant changes that its normal reactions are no longer given. 



130. The marking's of tiie cell-wall. Uniform thickening of 

 the whole cell-wall is extremely rare ; even in the examples 

 which are commonly given to illustrate it, pores or channels, 

 more or less distinctly visible, interrupt 



its continuitj'. 



131. The thickenings may possess 

 great irregularity, or thej' may be so 

 strictly localized and regular as to form 

 characteristic features of the widest use 

 in diagnosis. They ma}' pi'oject out- 

 wardlj', forming ridges, spines, and 

 other sculpturings ; or, as is most com- 

 monlj' the case, inwardlj', giving rise 

 to rings, spirals, etc. 



132. If tlie wall is thickened through- 

 out, except at well-defined points, de- 

 pressions or pits are produced, varying 

 considerably in outline, but occurring 

 generall}' as simple dots or lines. In 



some cases it is not difficult to see that these dots or lines are 

 true pores or fissures running from one cell to the next. 



' Accoi'ding to Schinitz, the cell-wall is produced by the conversion of the, 

 limiting film of protoplasm into cellulose. That the cell-wall is formed at the 

 limiting film admits of no question. 



Fig. 3. Pitted duct; from stem of Ciohorium Intybus. (Jacobs.) 



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