444 



THE STRUCTURE OF ECONOMIC PLANTS 



epidermal cells that are to become fibers begin to swell slightly; 

 and at the end of twelve hours, they have a bulbous appearance and 

 the cells are more vacuolate. (Fig. 12.7, A.^ After twenty-four 

 hours, the tubular fiber cells are much elongated and have usually 

 attained their full diameter. (Fig. xxy, B.) By the second day, 

 there is a differentiation of the fibers so that some are longer with 

 pointed tips, while others are short and rounded. At the end of 

 three days, they appear definitely tapered for approximately a 

 quarter of their total length. At this time, the cells have large 



:i^ f% 



(^1 



miMmm 



Fig. L17. A, transection of epidermis of ovule on day of flowering showing origin of 

 cotton hairs ; B, same, 14 hours after opening showing elongation of hairs. The variety is 

 Mexican Big Boll. (Photomicrographs by Lang.) 



vacuoles and the walls are still thin, since during the entire period 

 of elongation the protoplast of the hair is enclosed only by a pri- 

 mary wall containing pectic substances, cellulose and probably 

 waxes. Anderson and Kerr state that there is no evidence of cutin 

 at this time, but that there is "a coherent skeleton of cellulose from 

 the first day of its appearance." This is in contrast to reports by 

 other investigators [Farr and Eckerson; Hess, Trogus and Wergin; 

 Sakostschikoff, Korsheniovsky, and Rutikoly — reviewed in 

 Anderson and Kerr (i)] that cellulose does not appear in the 

 walls until from five to as many as thirty-six days after flowering. 

 This difference may be accounted for on the basis of the small 

 amount of cellulose present, and its effective insulation by pectic 

 substances. 



