ELEMENTS OF ORGANIC STEUCTtTEE. 



227 



nucleus, but ai-e filled with a sap or juice containing citric 



acid and sugar. 



Li the pith of the rush, star-shaped cells are found. In 

 common mould the cells are long and 

 thread-like. In the so-called frog-spittle 

 they are cylindrical and attached end to 

 end. In the hark of many trees, in the 

 stems and leaves of grasses, they are 

 square or rectangular. 



Cotton-fiber, flax and Iiemp consist of 

 long and slender cells, fig. 31. Wood is 

 mostly made up of elongated cells, tapered 

 at the ends and adhering together by 

 their sides. Fig. 49, c. h., p. 271. 



Each cotton-flber is a siugle cell which forms an 

 external appendage to the seed-vessel of the cot- 

 ton plant. "When it has lost its free water of 

 vegetation and hecome air-drj', its sides collapse 

 and it resembles a twisted strap. A, in fig. 31, 

 exhibits a portion of a cotton-fiber highly magnified. 

 ^ The flax-fiber, from the inner bark of the flax- 



^' " ■ stem, b, flg. 31, is a tube of thicker walls and 



smaller bore than the cotton-fiber, and hence is more durable than cot- 

 ton. It is very flexible, and even when crushed or bent short, retains 

 much of its original tenacity. Hemp-fiber closely resembles flax-fiber in 

 appearance. 



Xliiclceniiig of tlie Cell-]Weiinl>raiie. — The growth of the 

 cell, which, when young, always has 

 a very delicate outer membrane, often 

 results in the thickening of its walls 

 by the interior deposition of cellu- -^ 

 lose and lignin. This thickening may ^^, 

 take place regularly and uniform- S^ 

 ly, or interruptedly. The flax-fiber, (' 

 6, flg. 31, is an example of nearly ( 

 uniform thickening. The irregular ' _ 



deposition of cellulose is shown in ^ ~Y ^^ Nr 

 fig. 33, which exhibits a section from Fig. 33. 



the seeds (cotyledons) of the com- 

 mon nasturtium, {Tropceoluni majus). The original membrane is coated 

 interiorly with several distinct and successively-formed linings, which 

 are not continuous, but are irregularly developed. Seen in section, the 



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