PLANT CELL GROWTH AND NUTRITION' 



479 



Figure 10. Electron-microscope studies of the cellulose in the wall of Valonia. A: 

 Randomly arranged fibrils in the newly formed wall on the surface of a sporeling. 

 B: Strongly oriented, parallel fibrils in the mature wall of an old vesicle. Strands can 

 be seen in three directions in successive lamellae. (From Steward and Muhlethaler, 

 1953.) 



orderly way described? This is an example of the problems that the 

 molecular architecture of complex substances presents, and it is in an 

 understanding of the organization of the living system that a solution 

 should be sought. 



In part, the controls over these events may be environmental. If 

 sporelings grow as filaments, away from red light ( as in V. ocellata, cf. 

 Steward, 1939), their cellulose chains tend to run along the length of 

 the filament. One suspects that if sporelings could be grown in a sym- 

 metrical environment, with the geotropic stimulus neutralized and 

 without diurnal variations in light and temperature, much of the struc- 

 ture of the normal cellulose wall would change. Anderson and Kerr 

 ( 1938 ) have shown, very beautifully, that the wall of the cotton hair, 

 which forms one new layer each day, becomes much more uniform and 

 homogeneous if the diurnal variations of the normal environment are 

 replaced by constant conditions (Figure 11). Although very little work 

 has yet been done upon them, it is significant that the newly formed 

 walls of free carrot cells, growing in a liquid medium under uniform 

 illumination and aeration, are, hke the aplanospores of Valonia, formed 

 of cellular fibrils arranged in a relatively random fashion ( Figure 12 ) . 

 However, in the course of normal development of more organized 

 structures— such as fibers and vessels— which obviously develop under 

 the influence of asymmetric stimuli within the plant body, the cellulose 

 wall has the strongly oriented structure described by so many workers. 



