84 THE COTTON PLANT IN EGYPT chap. 



The cessation of this growth is thus the result of 

 internal constitution, though environmental changes 

 exercise a limited effect. (Fig. 50 also, "310" and 

 " 77 ".) 



It appears, though this point is not quite clear as yet, 

 that linear extension has ceased before thickening of the 

 wall commences. It is this thickening which determines 

 the strength of the individual fibre. The strength of the 

 commercial sample depends not o]ily on thickness, but also 

 on uniformity of strength as between different fibres. 

 Moreover, commercial strength does not merely result 

 from the thickness of the cell-wall, but also from the 

 uniformity of that thickness over the whole length of 

 the cell, and possibly is also affected by the "texture"' 

 of the thickening layers. 



Even as regards the simple fact of thickening, we find 

 many curious delusions in the extant literature of the 

 subject. Confining ourselves to the known data, the 

 matter is a simple one ; concentric layers of cellulose, 

 probably delimited from night to night, are laid down on 

 the interior of the delicate cellulose-cuticle wall, until a 

 certain thickness is reached. This deposition is not uniform, 

 but results in the formation of simple pits at intervals, 

 elongated obliquely. 



In consequence of this pitting we find that fibres devoid 

 of secondary thickening show no twisting when extricated 

 from the unripe boll and dried, while fibres taken from a 

 boll which is nearly ripe exhibit rapid and uniform 

 twisting as they dry, owing to the closure of the solid 

 portions of the cell-wall into the minute spaces formerly 

 occupied by the pits. 



Strength. — In respect of the strength of the lint, we 

 have some evidence to show that the causes which provoke 

 shedding are simultaneously effective in weakening the 

 lint. 



The lint from the terraces (Fig. 47) in 1909 was 



