Growth-rings in the Cell Wall of Cotton Hairs. 549 



At this stage, in hairs which had not swollen to the extent described above, 

 very fine transverse wrinkling of the cuticle was seen, indicating that the 

 transverse swelling of the secondary wall is accompanied by longitudinal 

 contraction. This fact, which is indeed a matter of common experience 

 with other reagents such as NaOH, propounds a question as to the space- 

 dimensional internal structure of the cellulose wall, which has not been 

 adequately answered. 



The contraction is also demonstrated by the form which the remains of 

 the protoplasm assume, since measurements show that this part of the hair 

 is not appreciably swollen by the treatment, and yet it is evidently put under 

 longitudinal compression. It follows that the cellulose wall in every degree 

 of hydration, and presumbly, therefore, in its original untreated state, has 

 some internal construction which is not the same in a tangential direction as 

 is the case radially. 



The swollen wall found in the great majority of the hairs at this stage is 

 sometimes homogeneous, within the limits of microscopy, and sometimes 

 shows a denser zone whereby the wall is divided into two portions. The 

 outer of these two is the wider. No cases of more than two such portions 

 have been noticed in the material examined, though it is probable that one 

 or two more might be found exceptionally. Assuming that these layers 

 might well be due to the growth of successive nights, and be demarcated 

 from each other at the daily growth stoppage brought about by the sunshine 

 effect, we may state that the average number of growth-rings at this stage 

 is about 1"0 ; some hairs have two, while some have none, A variation of 

 two days, plus or minus, in the dating of these events in the life history of 

 the fruit is quite likely at this stage ; the full maturation of the fruit in this 

 strain has a P.E. of +3"0 per cent, which allows an extreme chance variation 

 of 2J days either way at this present stage ; in other words, the number of 

 growth-rings may be expected to vary over a range of five, even if the indi- 

 vidual hair-cells are no more variable than the massive tissue structure of 

 the fruit. 



Thirty-day Fruit. — The appearance of the fuzz-hairs at this stage is most 

 strikingly different from that of their younger predecessors. The occasional 

 dense layer seen in the swollen wall at 27 days is now multiplied, and these 

 layers have become the most conspicuous part of the object, on account of 

 their smooth curves. The first growth-ring is still quite twice the thickness 

 of its successors ; on tracing it to places where it passes under a girdle of 

 cuticle there seems reason to believe that this extra thickness is not due to 

 less constraint on its swelling, but that it was initially a thicker deposit of 

 cellulose than its successors. 



