248 



HOW CROPS GROW. 



incapable of further growth, they possess neither proto- 

 plasm nor nucleus, but are filled with a sap or juice con- 

 taining citric acid, sugar and albuminoids. 

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

 In common mold the cells are long and 

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

 (algce) they are cylindrical and attached 

 end to end. In the bark of many trees, 

 in the stems and leaves of grasses, they 

 are square or rectangular. 



Cotton-fiber, flax, and hemp 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. See also Fig. 49, c, h, p. 292. 



Each cotton-fiber is a single cell which forms an 

 external appendage to the seed-vessel of the cotton 

 ^ plant. Wlieii it has lost its sap and become air-dry, 

 its sides collapse and it resembles a twisted strap. 

 \A, in Fig. 31, exhibits a portion of a cotton-fiber 

 highly magnified. The flax-liber, from the inner 

 bark of the flax-stem, b, Fig. 31, is a tube of thicker 

 walls and smaller bore than the cotton-fiber, and. 

 hence is more durable than cotton. It is very flexi- 

 ble, and even when crushed or bent short retains much of its original 

 tenacity. Hemp-fiber closely resembles flax-fiber in appearance. 



Thickening of the Cell-Membrane. The growth of the cell, which, 

 When young, has a very delicate outer 

 membrane, often results in the thick- 

 ening of its walls by the interior dep- 

 osition of cellulose and woody mat- 

 ters. This thickening may take place vJ-| 

 regularly and uniformly, or interrupt- 

 edly. The flax-fiber, b, Fig. 31, is an ex- 

 ample of nearly uniform thickening. 

 The irregular deposition of cellulose is 

 shown in Fig. 32, which exhibits a sec- 

 tion from the seeds (cotyledons) of the 

 common nasturtium ( Tropveolum 

 majus). The original membrane is coated interiorly with several dis- 

 tinct and successively-formed linings, which are not continuous, but 

 are irregularly developed. Seen in section, the thickening has a waved 

 outline, and, at points, the original coil-membrane is bare. Were these 

 cells viewed entire, we should see at these points, on the exterior of 

 the cell, dots or circles appearing like orifices, but being simply the 



Fig. 31. 



Fig. 32. 



