148 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



crossed Nicols, and it now seems probable that this is due, to some 

 extent at least, to the presence of cellulose micelles inclined in a spiral 

 less steep than that in the central layer, though this was not the original 

 interpretation. 



It is interesting to notice, in view of these differences of structure 

 between collenchyma cells and the other cells we have looked at in this 

 chapter, the very special function which appears to be confined largely 

 to these cells. Normally collenchyma is produced early in the develop- 

 ment of the stem and it is generally considered that its chief role is to 

 provide mechanical strength in such a way as to impede longitudinal 

 growth as little as possible. It would seem that this is achieved through 

 the presence of cellulose chains oriented almost longitudinally, giving 

 high ultimate strength in the longitudinal direction, coupled with a high 

 proportion of non-crystalline cellulose which will thus allow consider- 

 able extension. 



Cotton hairs 



Finally, attention must be called to the very important work which 

 has been going on for many years in different laboratories throughout 

 the world, on a variety of plant hairs. In some way these form more 

 suitable material on which to make growth studies in terms of wall 

 structure since they share with the algae the advantage that their 

 growth is less confined by the presence of neighbouring tissues. In 

 point of fact, detailed observations have been made on only two types 

 of hair. These may be exemplified by the staminal hairs of Tradescantia 

 studied by Martens and by van Iterson, and cotton hairs which have 

 naturally been the subject of much wider investigation. For reasons of 

 space, attention here will be confined to cotton hairs on account of 

 their greater familiarity and importance. The structure of the other 

 hairs is in essence very similar to that of cotton which, in turn, we shall 

 find in many respects to resemble rather closely that of the other elon- 

 gated cells we have reviewed here. A valuable summary of the more 

 botanical aspects of the numerous investigations carried out on cotton 

 has recently been presented by Flint (75). 



The pioneer investigations were made by W. J. Balls on varieties of 

 cotton grown in Egypt and, although his observations made in Egypt 

 many years ago have naturally been considerably extended, in the main 

 his interpretations still stand. Cotton hairs take the form of very long 

 (up to a few centimetres) threads with thick walls proliferating from the 

 epidermis of the seed. In transverse section the walls show little struc- 

 ture, but if they are stained in a substantive dye and particularly if 



