PLANT CELL MEMBRANES 273 



end bondage of the cellulose particles is strong. In the cells of the Avena 

 coleoptile, where rapid cell elongation takes place, these end-to-end bond- 

 ages are weak enough to be broken during this growth process; in the 

 mature, elongated membrane, the single rows of particles are arranged 

 side by side along the long axis of the cell in "barrel-hoop" fashion. In 

 this connection it may be pomted out that in all the cells studied to date, 

 the burden of swelling, plasticity, and elasticity has rested upon the inter- 

 crystalline material. When extreme plasticity and absorptive properties 

 are desired, and strength is not at a premium, as in the root hab, the cellu- 

 lose is often excluded entirely from the construction of the cell membrane. 

 When strength as well as flexibility is desired, the colloidal materials, such 

 as pectin and protein, are reduced m quantity; in some instances they are 

 rendered more impervious to swelling agents by compound formation {e.g., 

 calcium or sodium pectates), and are intimately blended with cellulose 

 particles to produce the desired physical and chemical combination. 



Formation of the Cellulose Particles 



The findings up to this point had furnished no clue to the method of 

 elaboration of cell membrane materials in the living protoplasm. Small 

 cells, such as the cotton fibers, required that the observer look through the 

 membrane in order to see protoplasm during its period of synthetic activity. 

 Protoplasm expressed from such cells tends to produce artifacts by coagula- 

 tion and other physicochemical changes; optical sectioning of carefully 

 mounted fibers revealed nothing definite in these cells as long as an entirely 

 unknown process was involved. The large, one-celled alga, Valonia ventri- 

 cosa, had been exammed and reported upon at one of the meetings of the 

 American Chemical Society.^' Earlier workers had been attracted to the 

 study of Valonia because of its highly crystalline, well-oriented membrane. ^ 

 We were particularly interested in studying it because of the ease with 

 which its protoplasm could be removed and maintained in good condition 

 for microscopic examination. Arrangements were made, through the cour- 

 tesy of the Carnegie Institution of Washington, to obtain fresh samples of 

 Valonia ventricosa, V. macrophysa, and a closely related form, Halicystis 

 osterhoutii from their Dry Tortugas laboratory and the Bermuda Biologi- 

 cal Station for Research, Inc. Preserved samples of Halicystis ovalis were 

 furnished from the beaches of Pacific Grove, California, by Dr. George J. 

 Hollenberg. 



The mature membrane of Halicystis was found to contain a high propor- 

 tion of non-cellulosic material. Through this colloidal matrix the cellulose 



Figure 106. Cotton fibers grown in constant light. Lamellae about 1 /z wide. A, 

 15 days, 1 lamella, 2nd forming (920 X). B, 49-day cross section; 6 cellulose particles 

 with micrometer scale (1 space equals 2.35 m measured from center to centerof adjacent 

 Unes). C, E, F, 49-day cross sections at middle. D, 60-day dry, cut in cork (460 X, 

 enlarged to 890). G, X-ray diffraction pattern of fibers matured in constant Ught. 

 H, In daylight. 



