THE COTTON HAIR 245 



an average thickness of 0-4 /x each, corresponding with the number 

 of days from the cessation of growth in length. 



In the dead and dried condition the lumen of the cotton hair 

 contains the remains of the cytoplasm and nucleus responsible for 

 its growth w^hen alive. The whole of the lumen is covered with 

 a layer of cytoplasm, whilst the centre is occupied by vacuoles 

 containing cell sap, the osmotic pressure of which keeps the wall 

 turgid. One of the points of greatest controversy in regard to the 

 structure of the cotton hair is the presence of so-called pits. Balls 

 described oblique slit-like cracks in the secondary thickening, 

 tapering from the centre of the hair to the outside, and he claimed 

 that they were responsible for the convolutions of the hair. 



Fig. 82. — Normal cotton hair, showing convolutions and thickened edge. 



(From Denham, Shirley Inst. Mem.) 



Denham considered that thev were due to a double line of weakness 

 recurring in two or more super-imposed layers. 



A number of different writers have described the presence of 

 spirals and striations on the cell wall of the hair. This aspect of 

 the morphology of the cotton hair has been widely studied by 

 Denham, who claimed that the spirals and striations could be 

 interpreted from the behaviour of the cytoplasm in the hairs of 

 Tradescantia. Here the striations directly follow the movement 

 of the cytoplasm, and particles can be seen moving along adjacent 

 striations in parallel and opposite directions. There is therefore 

 no reason for giving up the view that these striations are due to a 

 localised spiral thickening which reaches its highest expression 

 in the structure of tracheids and vessels. For a very full discus- 

 sion, the original paper by Denham should be consulted. 



The convolutions of the hair have also been investigated by 

 Denham at the Shirley Institute. These may be divided into four 

 classes : normal, movable, preformed and suppressed. Their 

 presence exercises a marked influence on the spinning j^roperties 

 of the hair. The normal and movable convolutions are due to the 



