446 THE STRUCTURE OF ECONOMIC PLANTS 



to the periodic deposition of the lamellated secondary wall. In 

 this connection, Balls stated that 



"the secondary thickening of the wall proceeds intermittently under 

 normal Egyptian field crop conditions, being arrested each afternoon; 

 that the cellulose of the hair consequently consists of a number of 

 concentric shells, layers, or 'growth-rings,' each one representing one 

 day's growth, with the exception of that of the primary wall; and 

 that the so-called fuzz-hairs are analogous with the lint-hairs, though 

 their growth-rings are coarser and more sharply demarcated." 



In describing the structure of the walls he found that the 



"hairs are covered, outside the cellulose of the primary wall, by a cuti- 

 cle, bearing wax, which is structurally . . . identical with the cuticle 

 of the testa, while it is structurally and chemically distinct from the 

 cellulose. The secondary wall, but not the primary, is traversed 

 obliquely to the hair axis by simple pits which are rarely visible except 

 in the living hair, and to these pits is due the twisting of the hair and 

 its characteristic convolutions after death." 



The death of the cell occurs following the opening of the boll, with 

 the resultant loss of water which causes the fiber cells to collapse, 

 forming flattened tubes with many twists. 



Kerr found that during the period of secondary wall formation 

 there are two lamellae laid down each twenty-four hours which 

 together constitute a daily growth ring. These two zones differ 

 in their porosity, the compact zones being deposited during the 

 day and the porous ones at night. The differences in porosity, 

 which result in the lamellated appearance of swollen fibers, are 

 correlated with temperature fluctuations, and the contrast between 

 adjacent porous and compact lamellae is greater when the night 

 temperature falls below io° C, while there is less differentiation 

 between the zones when it does not fall below 1.2.° C. Thus, while 

 Kerr confirmed Balls' observations regarding the diurnal deposition 

 of the growth rings, they are not in agreement with respect to the 

 method by which this is accomplished. Balls suggested that the 

 ring was formed in the night and early morning, and that growth 

 stopped entirely during the hot afternoons; while Kerr attributes 

 the rings to the formation of a continuous matrix of cellulose laid 

 down in different densities. 



Anderson and Moore (x) investigated the effect of continuous 

 light on the structure of the walls of fibers and coUenchyma and 

 found that the reaction of the two tissues to this factor is quite 



