SINNOTT: CELL DIVISION AS A PROBLEM OF PATTERN 33 



most cases exactly opposite similar walls in the cells above and below, so that 

 in such a group of cells, as seen from its outer surface, four walls usually 

 meet at a point instead of three as in most tissues. The point where these cells 

 come together is evidently subject to a good deal of strain as the cells expand 

 and intercellular spaces thus develop here very early (Fig. 4). These become 

 much enlarged in the mature tissue. The relation between such a type of cell 

 division and the development of aerenchyma has previously been pointed out 

 by the writer. 



The surface cells which are to give rise to stomata undergo a remarkable 

 series of divisions. In a vertical row of cells, every second one is a stomatal 

 mother cell. The first division in it is longitudinal and usually unequal, with 

 the new wall convex toward the smaller daughter cell. The next, in the larger 

 cell, is convex in the opposite direction, so that a lens-shaped cell has now 

 been cut out with a larger cell on either side. The lens-shaped cell then divides 

 into two guard cells. 



This diversity in the type and direction of cell division in developing plant 

 tissue is of course not confined to Eqiiisctinn but is a familiar feature of the 

 process of differentiation in all multicellular plants. The important fact which 

 it emphasizes is that no single method of division is universal, and that every 

 "rule" is frequently broken. Evidently many factors may be concerned with 

 determining the plane of cell division. What a given cell will do depends not 

 upon some general principle of division, common to all cells, but upon the 

 conditions which exist at that particular place and time. Every cell is a part 

 of a general developmental pattern, and not only in the way it divides but in 

 every other aspect of its behavior it seems to be governed by its particular 

 place in that pattern. Driesch nearly half a century ago summed this up in 

 his famous aphorism that "the fate of a cell is a function of its position," and 

 Vochting many years before said the same thing in almost the same w'ords. 



This general fact of development, so well illustrated by the controlled 

 diversity of mode of cell division in the meristematic tissues of plants, should 

 be recognized by all students of morphogenesis. In a search for the mechanisms 

 which operate in the remarkable processes of organic development, we tend 

 to oversimplify the problem and to postulate factors which have a specific 

 method of operation. Thus the role of auxin, of light, or of a given gene is 

 often assumed to be a definite and invariable one, whereas its effect actually 

 is dependent in very great measure on the internal and external environment 

 in which it operates. In an eagerness to find specific organ-forming substances 

 and stimuli we have too often neglected the complex reaction system, the 

 developmental pattern in which these must work. Knowledge about specific 

 factors is very useful and is rapidly accumulating, but far more important 



