192 The Phenomena of Morphogenesis 



tween neighboring cells and often become very long (Bloch, 1946). A 

 study of the origin of such idioblasts ( cells distinctly different from their 

 neighbors) (Fig. 8-4) may throw light on problems of cellular dif- 

 ferentiation ( Foster, 1956, and others ) . 



Stomatal initials are set apart by differential cell divisions ( Bunning and 

 Biegert, 1953 ) . A smaller, more densely cytoplasmic cell and a larger one 

 are formed by a late division of a surface cell. The former divides again, 

 this time equally and longitudinally, to form the guard cells, the contents 

 of which soon become markedly different from those of the other epidermal 

 cells. In monocotyledonous plants like this, where the cells are in regular 

 longitudinal rows, the stomatal initials are cut off at the ends of elongate 

 cells. In most dicotyledons, where the cells of the developing epidermis 

 are more nearly isodiametric, the initial is cut out of a corner of the cell 

 and divides again to form the guard cells. 



A number of cases have been reported where the differentiation of one 

 type of cell evidently induces changes in the character of adjacent ones. 

 Thus in Sedum there are groups of cells that form tannin, and in these 

 regions stomata do not differentiate (Sagromsky, 1949). In Potamogeton 

 roots those cells of the exodermis that are just under the already dif- 

 ferentiated trichocytes divide several times, unlike the other cells in this 

 layer, and so form groups of small cells, one below each trichocyte 

 ( Tschermak-Woess and Hasitschka, 1953b ) . In a species of Begonia where 

 there are silver spots on the leaf surface, there is a hair formed in each 

 spot save the very small ones, and the larger the spot, the longer the hair 

 (Neel, 1940). 



Cell Size (p. 29). Some of the most conspicuous differences between 

 cells are in their size. Meristematic cells in most cases are small, and after 

 the final division the daughter cells increase considerably in size. The 

 extent of this increase is determined by the time in development when 

 division ceases in that particular cell lineage and by the position of the 

 cell in the general histological pattern. Pith cells are usually large be- 

 cause they have had a long period of enlargement since their last division, 

 and epidermal cells relatively small since division there lasted longest. 

 Many size differences, however, such as those between the large vessels 

 of ring-porous woods and the small elements around them, are due to 

 local differential factors, since the cambial initials are alike. 



One type of cellular differentiation is unlike all others in that it in- 

 volves fundamental alteration of the cell itself. Many instances are now 

 known in which mature cells, aroused to division by various agents, are 

 found to have twice as many chromosomes (or more) as they did at 

 their last preceding mitosis. During the differentiation of such a cell 

 from meristematic condition to maturity a doubling of the chromosome 

 complement must have taken place. Such a change, though not directly 



