478 PLANT GROWTH AND PLANT COMMUNITIES 



exogenous supply of substances, of the kind present in coconut milk, 

 which induce growth by cell division. 



The idea that metabolic patterns may be different in rapidly pro- 

 liferating and resting cells has been brought out by Braun ( 1954 ) with 

 reference to the biochemistry of tumor and gall development. Refer- 

 ence has been made by Braun to the work of Neish and Hibbert 

 (1943), who showed that it is a primary feature of the tumor cell to 

 divert sugar to the formation of protein— a feature which is shared by 

 carrot cultures stimulated to grow by cell division by the effect of 

 coconut milk. 



Asymmetric synthesis and molecular architecture of complex sub- 

 stances. The asymmetry and organization of the living system and the 

 factors that control growth also exert an influence over the ways in 

 which complex substances are built up in the cell; this is amply illus- 

 trated by the well-documented case of cellulose in the cell wall. The 

 problem here is not only the formation of the chains of anhydro-glucose 

 units linked by 1-4 yS (D) gluco-pyranose linkages (this is one of in- 

 numerable asymmetric syntheses in the asymmetric environment of 

 cytoplasm ) , but to explain the formation of the molecular architecture 

 of the cellulose as it exists in the plant cell wall. This architecture must 

 be built up by events determined at a higher level of organization, for 

 its consequences are visible with the electron microscope. 



Cellulose newly formed at a naked protoplasmic surface consists 

 of fibrils which are randomly arranged, forming a tangled weft with 

 the minimum of organization. This is well shown by the newly formed 

 cellulose on the surface of a Valonia aplanospore— i.e., a spherical 

 droplet of naked protoplasm at the time of its formation (see Figure 

 10 ) . As the sporeling grows— and it does this best if it is stationary and 

 attached to a surface like marble, which simulates the natural sub- 

 stratum (Steward, 1939)— successive cellulose layers are laid down. 

 The cellulose chains become increasingly oriented, lying parallel to one 

 another as they follow a spiral path around the enlarging vesicle. But 

 the remarkable thing is that, after having first formed cellulose in this 

 way, the direction of the fibrils shifts abruptly— usually so that the 

 fibrils in the new direction cross those of the old at angles about 

 120°/60°. This process is repeated, so that in a multilayered wall like 

 that of Valonia ventricosa the fibril direction in the first, fourth, seventh, 

 etc. layers tend to be parallel ( see Figure 10 ) . 



Here we have, then, what seems to be morphologically a very 

 simple system— an expanding, globular vesicle with a central, sap-filled 

 cavity and a thin peripheral sheet of protoplasm which secretes a wall 

 around itself. What kind of message directs the vesicle to abandon the 

 first formed random wall and to produce the first spiral arrangement? 

 And what signal then prompts the periodic shift in direction in the 



