42 



THE CELL AND PROTOPLASM 



— dermatosomes, ellipsoid particles, fusi- 

 form bodies, elongated shreds or fibrils, etc. 

 During the last 100 rears a succession of 

 investigators have hypothesized that cer- 

 tain of these fragments are the elementary 

 units from which the cell wall is con- 

 structed. That such fragments are hetero- 

 geneous is indicated by their particle double 

 refraction, by their dichroism, and by other 

 physical evidence. Furthermore, it should 

 be clearly recognized that many cell walls 

 and wall layers are much less than one 

 micron in diameter and that the narrower 

 types of lamellae grade down to 500 A or 

 less in diameter. Obviously, these struc- 

 tures cannot be constituted of units 10,000 

 A or more in diameter. 



As previously stated, much less is known 

 about the primary wall of plant cells than 

 about the secondary. Evidence is accumu- 

 lating', however, which indicates that the 

 primary wall has a fundamentally similar 

 physical structure. Statements in the lit- 

 erature that the primary wall is amor- 

 phous or isotropic, that it is not comi^osed of 

 typical cellulose, that it is constituted of 

 unoriented cellulose, or that it is composed 

 of cellulose which is invariably oriented at 

 right angles to the long axis of the cell or 

 to the direction of major expansion are in- 

 correct and based upon errors of observa- 

 tion or of interpretation. 



The primary wall, like the secondary one, 

 is composed of a continuous system of more 

 or less extensively coalesced microfibrils of 

 anisotropic cellulose, or at times of chitin, 

 mannan, etc. The elongated porosities of 

 the cellulosic matrix commonly contain 

 pectic compounds and henucelluloses, and 

 not infrequently liguin, cutin, suberin, 

 waxes, and many other organic; substances. 

 The primary wall frequently is lamellated, 

 and, as in the case of the secondary wall, 

 the orientation of the microfibrils, micelles, 

 and chain molecules varies greatly in differ- 

 ent t3qies of cells and in different lamellae 

 or parts of the same wall. Growtli and 

 expansion of the primary wall appeal', in 

 many cases at least, to involve the pulling 

 apart of the fibrils in the previously foi'med 

 lamellae and the deposition of new layers 



of coalesced fibrils. In fact, I suspect that 

 the classical controversy regarding growtli 

 by intussusception versus growth by ap- 

 position will ultimately be settled largely 

 in favor of apposition. 



Although we are gradually obtaining a 

 much clearer picture of the complex struc- 

 tures of the walls of plant cells, we have 

 learned verj^ little during the last 100 years 

 about the actual processes by which these 

 remarkable structures are formed by the 

 protoplast. In this direction lies an un- 

 explored field for future research. 



References Cited 



Anderson, D. B. and Kerr, T. 1938. Growtli and 

 .Stiufture of Cotton Filter. Ind. Eng. Cliem. 

 Ind. Ed., 30: 48. 



AsTBURY, W. T., Marwick, T. C. and Bernal, 

 .T. D. 1932. X-rav Analysis of the Structure of 

 the Wall of Valoiiia ventricosa. Proc. Boy. Soc. 

 London, 109: 443. 



AsTBURY, W. T., Preston, R. D. and Norman, 

 A. G. 1935. X-vaj examination of the Effects 

 of Removing Non-cellulosic Constituents from 

 Vegetable Fibers. Nature, 84: 391. 



Bailey, I. W. and Kerr, T. 1935. The Visible 

 Structure of the Secondary Wall and Its Signifi- 

 cance in Physical and Chemical Investigations 

 of Tracheary Cells and Fibers. /. Arnold Arbo- 

 retum, 16: 273. 



— . 1937. The Structural Variability of 



the Secondary Wall as Revealed by "Lignin" 

 Re-idues. J. Arnold Arboretum, 18: 261. 



Bailey, I. W. and Vestal, M. R. 1937. The 

 Orientation of Cellulose in the Secondary Wall 

 of Tracheary Cells. J. Arnold Arboretum, 18: 

 185. 



. 1937. The Significance of Certain 



Wood-destroying Fungi in the Study of the 

 Enzymatic Hydrolysis of Cellulose. J. Arnold 

 Arboretum, 18: 196. 



Berkley, E. E. 1939. Cellulose Orientation. 

 Strength and Cell Wall Development of Cotton 

 Fibers. Textile Besearcli, 9 : 355. 



Bonner, J. 1935. Zum Mechanismus der Zell- 

 streckung auf Grund der Micellarlehre. Jnhrb. 

 IVis.^. Botan., 82: 377. 



Castle, E. S. 1936. The Double Refraction of 

 Chitin. ,/. aen. Physiol, 19: 797. 



. 1937. Membrane Tension and Orienta- 

 tion of Structure in the Plant Cell Wall. J. 

 Cellular Comp. Physiol., 10: 113. 



. 1938. Orientation of Structuic in the 



Cell Willi of IMiyconiyccs. J'rotoplasma, 'SI: 331. 



Cokren's, C. 1893. Zur Kenntnis der innercn 



Struktur einiger Algenmembranen. Beitr. Mor- 



phol. Physiol. Pflamens (Zimmerman), 1: 260. 



