PLANT CELL MEMBRANES 263 



seed coat. Early stages of fiber elongation from epidermal cells which were 

 apparently daughter cells of these recent divisions furnished direct evidence 

 that fibers may originate from cells which are not yet formed upon the date 

 of flowering. 



The absence of Hquid substance in the boll cavity which had been com- 

 monly designated "boll sap" removed the possibility of nutrition of the 

 developing fiber through such a medium, and indicated that the materials 

 for its growth are transported through its basal connection Avith the seed. 



The study of the formation of abnormalities in developing fibers showed 

 that while cell enlargement and the formation of a thick cell membrane 

 are two of the most conspicuous phases of cotton fiber growth, the tendency 

 to enlarge in an approximately linear direction is inherent. If obstructed 

 in one region of the boll cavity, however, the portion of the fiber concerned 

 appropriates any available space in the immediate vicinity for enlargement. 

 This may result in change in diameter, change in direction of growth, or 

 any one of many types of abnormality. In the three varieties of cotton, 

 Pima, Super Seven, and Acala, the measured densities of the fiber masses 

 were found to increase in the order named. Fiber abnormalities were 

 observed to increase in the same direction and a definite relationship was 

 suggested between these two factors. 



Broadening of the Experimental Approach 



During the progress of these studies it became evident that the advan- 

 tage of the cotton fiber, with respect to the large quantity of cell membrane 

 material, was not furnishing, under the experimental conditions employed, 

 additional information concerning the fine structure of the cell membrane 

 and the formation of membrane-building materials in the living protoplasm. 

 From the viewpoint of plant cells in general, the cotton fiber is one of the 

 most highly differentiated. Even among those cell membranes which 

 contain cellulose — and many do not — it would be classed as a specialized 

 and not a primitive type. The failure to observe finer details of membrane 

 formation and structure indicated a possible need for a broader attack upon 

 the problem, involving cells of various types throughout the plant kingdom. 

 Facts gleaned from such sources might then be used in attempting to under- 

 stand the development and structural problems of the cotton fiber. 



This task was undertaken by Mrs. Farr and Dr. Sophia H. Eckerson of 

 the Institute staff. Their first studies were made with the cellulose-forming 

 bacterium, Acetohacter xylinus. All the microscopic observations were 

 made without the use of the usual bacteriological methods of staining; 

 however, certain microchemical reactions and polarized light were used to 

 assist in the examination of individual organisms and chains of organisms 

 (Fig. 100). The cellulose layer surrounding the protoplast and the non- 

 cellulosic exterior layer of the bacterial organism were thus differentiated. 

 These procedures represented a, more direct approach to the physical and 



