THE STRUCTURE OF ECONOMIC PLANTS 



Secondary Wall Thickening. — As the cell differentiates, its 

 walls may be variously thickened and additional layers are depos- 

 ited against the primary wall. Bailey and Kerr (5) have described 

 the secondary wall of normal tracheids, fiber-tracheids, and libri- 

 form fibers as consisting of three layers : narrow outer and inner 

 layers, and an intervening one of variable thickness. (Fig. i.) 

 The outer and inner layers differ from the middle one in the orien- 

 tation of the cellulose fibrils, which are arranged at approximate 

 right angles to the long axis of the cell, while those of the mid- 



rw --'■' 7^^^ ^^^ layer are longitudinally or 



^^^^fejUgl^^^ ^g diagonally parallel to it. They 

 ' y^^^Sa^^^^^^SKL. ^ find, however, that deviations 



from the "typical 3 -layered type 

 of secondary wall are not of 

 infrequent occurrence. ' ' In many 

 thick-walled fibers, there appears 

 to be no differentiation of the 

 inner layer; and the middle layer 

 may consist of a series of lamellae 

 or growth rings. Kerr (14) has 

 shown for Gossypium that each 

 growth ring is comprised of two 

 lamellae, one porous and the 

 other compact, and that one of 

 these double rings is deposited 

 daily. (Fig. 3.) 

 The Pits. — The pits are thin spots in the cell wall which occur 

 at points where no secondary wall materials have been deposited. 

 In the formation of the secondary walls of two adjacent cells, the 

 wall substance is laid down in such a way that these spots coincide, 

 being separated only by the pit membrane consisting of the two 

 primary walls and the intercellular substance. A simple pit is 

 comprised of two opposed cavities with straight sides, which are 

 separated by a closing membrane; but such pits may be variously 

 modified with respect to size and shape, and their frequency and 

 distribution depend upon the nature of the tissue of which the cell 

 is a component part. In some instances, the secondary walls bound- 

 ing the pit project inwardly without contacting the membrane and 

 overarch the cavity to form a bordered pit. In such types, the cen- 

 tral portion of the primary wall or pit membrane may be thickened 



A 



Fig. 3. Transection of a cotton fiber 

 grown under field conditions, swollen in 

 cuprammonia, and subjected to pressure to 

 show the conspicuous pattern of the growth 

 rings. X550. (Photomicrograph by Anderson 

 and Moore, Ind. and Eng. Chem.) 



