Differentiation 199 



1. What determines the relative frequency of divisions on the inside of 

 the cambial initials to those on the outside, the relative amount of 

 xylem and phloem? 



2. What determines how cambium derivatives differentiate into the 

 widely diverse sorts of cells found in the mature tissues? 



3. What maintains so perfectly the anatomical pattern of the xylem 

 and phloem? 



Bannan and others (p. 81 ) have shown that many radial files of cells are 

 begun at the cambium and then die out and that many rays are initiated 

 only to disappear, the net result being a very precise distribution of rays 

 and vertical elements with reference to each other. The files of tracheids 

 remain at a constant width, and the rays are evenly spaced with reference 

 to each other, as can be seen in a tangential section of wood. These rela- 



Fig. 8-11. Portion of transverse section of leaf of Sanseviera. Bundle of fibers be- 

 ginning to differentiate in the midst of fundamental tissue. Compare with Fig. 8-10. 

 ( From Meeuse. ) 



tionships are so constant and specific that they are used as taxonomic 

 characters. 



This same problem of a specifically patterned distribution of structures 

 meets us in many other places, such as in the spacing of bundles in cross 

 sections of the stems of monocotyledons, of stomata in the leaf epidermis, 

 of root hairs, or of developing sclereids in the cortex. Biinning has ex- 

 plored this problem (1948, pp. 173-179). He suggests that a specific 

 developing structure prevents the differentiation of another like it within 

 a certain distance of itself and cites some experimental evidence in sup- 

 port of this idea (Biinning and Sagromsky, 1948). In the cells immediately 

 around a young stoma initial, the nuclei always lie on the side of the 

 cell next to the initial, as if in response to a chemotactic stimulus (Fig. 

 8-12). A few cells farther out, they have normal positions. Biinning be- 

 lieves that a hormonal substance passes out from the young stomatal cells 

 which stimulates cell division, as shown by the production of accessory 



