78 THE DEVELOPMENT OF EAW COTTON 



directions as they dry up, but hairs of the twenty-four 

 day twist up as they dry. By the twenty-seventh day 

 the wall is visibly thickened with secondary deposits of 

 cellulose on the interior of the primary cellulose wall and 

 its cuticle. This accession of material continues until the 

 boll is about to crack, but the most rapid increase is 

 noted about the thirty -sixth to thirty-ninth days. 



Meanwhile, by the thirty- third day it is easy to dis- 

 tinguish the simple pits in the wall (Fig. 12). These pits 

 are common in many kinds of vegetable cell-wall, and 

 are not in any way peculiar to cotton. They are of 

 about the same length as the diameter of the cell, and 

 are set at an angle of about 30 degrees to the axis of the 

 hair. 



They are not at all easy to recognize; ordinary illumination 

 with a good microscope does not display them, owing to the 

 translucency of the cell wall and the background of granular 

 protoplasm. A much higher and better magnification (e.g., 

 Zsiss compensating ocular 6, and 3 millimetre apochromatic 

 objective) is needed than their size would appear to warrant, 

 together with "critical" illumination. It should also be 

 noticed that they cannot be seen in ripe hairs, having been 

 obliterated by the twist, nor are they distinct in old pickled 

 material ; but they cannot be missed in the unripe, untwisted 

 hairs, given a good microscope, new material, and correct 

 illumination. 



That they have thus far escaped notice is probably due 

 to the fact that the observers who had the microscopes 

 had not the material, and vice versa. The discovery of 

 them renders unnecessary a great deal of speculative 

 philosophy which has been accumulated round the sub- 



