74 THE DEVELOPMENT OF RAW COTTON 



continues to increase until the twenty-fifth day of de- 

 velopment, after which (Fig. 13) its wall begins to thicken, 

 giving strength to the lint. This thickening is not uni- 

 form, but leaves simple pits in the wall (Fig. 12), set 

 obliquely, and the closure of these pits when the wall 

 dries after the boll opens gives twist to the fibre. The 

 uninucleate cell-contents remain alive until the boll 

 begins to open, when they die through desiccation. 



The ordinary epidermal cell of the outer ovule coat has 

 a thick basal wall, separating it from the subepidermal layer, 

 thinner side-walls, a thin cuticle covering the outer wall and 

 dipping slightly between the side-walls, with a nucleus 

 which is about one-fifth of the length of the cell, and small 

 sap cavities in the protoplasm. 



The outer wall of some of these cells bulges, their proto- 

 plasm becoming densely granular in the protuberance, while 

 the nucleus moves up closely behind these granules. The 

 swelling enlarges to about twice the diameter of the original 

 cell, and the nucleus passes out into it with all the chromatin 

 retracted into a large, deeply-staining, central nucleolus. 

 The nucleus keeps at a short distance behind the tip of the 

 swelling, or hair, which continues to elongate at the rate of 

 about 1 millimetre a day, which may be expressed in terms 

 of diameter as about twice the diameter added to the length 

 on the average every hour. It is almost certain that in the 

 case of Egyptian cotton this elongation is not continuous, 

 but is intermitted during sunshine in the same way as the 

 growth of the branches or stem . 



The nucleus at a later stage appears to settle nea the 

 centre of the fibre, about one-third of the way from the tip. 



The cell wall remains extremely thin for the first three weeks, 

 and the cuticle which still covers it can scarcely be differ- 

 entiated, unless the wall has been swollen by Schweitzer's 

 reagent, when (baing unaffected by the ammoniacal copper 



