THE WALLS OF PLANT CELLS 



37 



swerino' these questions are not available at 

 present, it seems likely that the latter alter- 

 native may ultimately prove to be the cor- 

 rect one. 



It should be noted in passing that in 

 cases where the orientation of the macro- 

 fibrils or microfibrils, and of the chain mole- 

 cules, has been accurately determined^ in 

 the same material, the chain molecules are 

 oriented approximately parallel to the long- 

 axis of the fibrils. Apparent deviations in 

 the orientation of chain molecules or of 

 micelles Avithin the secondary wall are com- 

 monly due to fluctuations in the orientation 

 of fibrils, rather than to aberrations in the 

 alignment of chain molecules or. of micelles 

 within the fibrils. 



Not all of the visible concentricities of the 

 secondary wall are due, necessarily, to 

 fluctuations in density or porosity of the 

 cellulosic matrix. Thus, the broader con- 

 centricities that are of common occurrence 

 in tracheary and sclerenehymatous cells are 

 due largely to: (a) var.ying amounts of 

 lignin, polyuronide hemicelluloses, or other 

 organic substances that are deposited within 

 the porosities of successively formed parts 

 of the secondary wall; (h) the occurrence 

 of non-cellulosic layers in certain specific 

 types of cells (Figs. 16, 20) and (c) varia- 

 tions in different layers of the secondary 

 wall, in the orientation of the microfibrils 

 and pari jmssu of the chain molecules of 

 cellulose (Figs. 14 and 18). 



In many cases fluctuations in the porosity 

 of the cellulosic matrix, in the amounts of 

 organic substances that are deposited or 

 formed within the capillary spaces, and in 

 the orientation of the chain molecules and 

 microfibrils, occur simultaneously within 

 the limits of a single secondary wall. Much 

 of the controversy in recent years and many 

 of the existing discrepancies in the litera- 

 ture about the physico-chemical structure of 

 plant cell walls, and particularly that con- 

 cerning the orientation of chain molecules 



3 For example, in the case of VaJonia, compare 

 the work of Correns and others with that of Ast- 

 bury and his coworkers; or in the case of cotton, 

 compare that of Anderson and Kerr with that of 

 Berkley. 



and of micelles within them, might have 

 been avoided if the significance of the nu- 

 merous biological variables in the material 

 under invest igjit ion had been clearly 

 visualized and accurately interpreted. 

 Where a wall is composed of layers which 

 differ markedly in thickness (Fig. 14), or of 

 nudtiple layers of more nearly uniform 

 thickness but of widely fluctuating fibrillar 

 orientations (such as certain types of scler- 

 enchyma), it is frequently difficult to ob- 

 tain an accurate picture of its structure by 

 examining the wall in surface view with 

 polarized light, or by x-ray analyses of en- 

 tire cells. Thin transverse, longitudinal, 

 and diagonal sections provide the most 

 favorable material for preliminary investi- 

 gations. Not only may such sections be 

 examined under a polarizing microscope 

 both before and after extraction of non- 

 cellulof^e constituents with suitable sol- 

 vents, but they may also be swollen, in 

 order to reveal variations in density or 

 porosity of the cellulosic matrix and in the 

 swelling anisotropy of successive layers. 

 Furthermore, crystals of iodine may be in- 

 duced to form in the elongated porosities of 

 the cellulosic matrix (Fig. 22). Since these 

 crystal aggregates are oriented approxi- 

 mately parallel to the long axis of the 

 fibrils, they provide a convenient means 

 of studying variations in the orientation 

 of cellulose in different parts of a secondary 

 wall. Additional evidence may be obtained 

 by observing changes in the orientation of 

 "slip planes" and of predetermined planes 

 of hydrolysis (Fig. 21). The orientation 

 of pit apertures and of planes of mechanical 

 cleavage (Fig. 23) must be utilized with ex- 

 treme caution, particularly in dealing with 

 thinner types of secondary walls. 



In the four types of cells illustrated in 

 Figs. 14, 16, 18, and 20, the evidence which 

 may be obtained from these varied lines 

 of investigation is in close agreement, and 

 clearly indicates that the secondary walls 

 have the general types of structure that are 

 shown in Figs. 13, 15, 17, and 19. The cen- 

 tral layer of Fig. 16 and the dark layers of 

 Fig. 20 are isotropic in longitudinal and 

 diagonal sections of the cells, as well as in 



