82 THE COTTON PLANT IN EGYPT CHAP. 



thumb, and finger, of the expert, who objects to handling 

 even single-plant samples. Probably a combination of all 

 these methods with a minimum of microscopic observation 

 which is undesirable on account of its high subjective 

 error, will give us a line of attack, and we may be able in 

 the future to explain why a fibre has become strong or 

 weak, and how such weakness or strength is affected by 

 the environment. 



Even such an obvious character as length of fibre is 

 very difficult to examine during development. The tangle 

 of lint hairs on the seed is almost inextricable, especially 

 in the early stages, when the individual fibres are too 

 weak to endure combing. Further, as in the excavation of 

 roots, a fruit once dissected for examination can be no 

 longer observed, and averages must be struck. 



The little which we know may be summarised under 

 the microscopical evidence, with the evidence derived from 

 fluctuation and correlation. 



Cytology of the fibre. The development of the 

 fibre begins before fertilisation is accomplished, by radial 

 growth (Figs. 3, 5) of a large number of the epidermal cells 

 of the seed coat. These cells (Fig. 4) differ in no respect 

 from their neighbours, and it seems possible that the 

 density of the coating may be determined by the external 

 conditions during a day or two after flowering. Possibly 

 irregularity in length may arise from distribution of the 

 normal simultaneous "sprouting" of these cells over 

 several days. 



The young fibre (Figs. 6, 7) at once assumes its fibre 

 diameter, which is about twice that of the unaltered cell. 

 It remains unicellular throughout its career, and is always 

 covered by the cuticle which protected the original cell. 

 The familiar " beading " which follows treatment with 

 ammoniacal copper hydroxide is simply due to constriction 

 of the swelling cellulose by the cuticular remains. For 

 the first day the nucleus lies at the tip of the swelling, but 



