244 BOTANY 



coat has a thick basal wall, which separates it from the sub- 

 epidermal layer. The side walls are thinner, and a thin cuticle 

 covers the outer wall and dips s^lightly between the side walls. A 

 nucleus, about one-fifth of the cell in length, is also present, and 

 small vacuoles are to be found in the cytoplasm. The outer wall 

 of the cell bulges and the nucleus moves forward towards the tip. 

 The swelling of the outer wall increases to about twice the 

 diameter of the original cell, and the nucleus now stains deeply 

 and shows a well-marked nucleolus. The nucleus keeps close 

 behind the tip of the swelling or hair, which continues to elongate 

 at the rate of about 1 mm. per day. It would appear that 

 in Egyptian cotton this elongation is not continuous, but is 

 intermittent during sunshine. At a later stage, the nucleus 

 seems to settle near the centre of the fibre, and the cell 

 wall remains extremely thin for the first three weeks, and 

 the cuticle covering it can scarcely be distinguished unless the 

 wall has first received a preliminary treatment with Schweitzer's 

 reagent. 



The particular features of the hair which require special con- 

 sideration are : (1) General conformation, (2) primary wall, 

 (3) secondary thickening, (4) central canal, (5) pits in the hair 

 wall, (6) spiral markings and striations, (7) convolutions, and 

 (8) abnormalities. 



Like the epidermal cells of most plants which have their outer 

 wall differentiated into cuticle, the cotton hair is also cuticularised 

 on its outer surface. The cuticle proper in normal untreated 

 hairs is indistinguishable from the primary wall which is laid down 

 during the process of growth in length. The chemical nature of 

 cutin is still unknown, but it appears to be of a fatty or waxy 

 nature. The primary wall is composed of cellulose, and is 

 chemically distinct from that of the secondary deposition. Accord- 

 ing to Balls, " secondary thickening " in the hairs takes place by the 

 laying down of concentric layers of cellulose, probably delimited 

 from night to night, on the delicate cellulose-cuticle wall until a 

 definite thickness is reached. By means of swelling reagents 

 (9 per cent. NaOH, followed by carbon disulphide). Balls was able 

 to prove the presence of such rings up to the number of 25, with 



