SINNOTT: CELL DIVISION AS A PROBLEM OF PATTERN 



31 



Such divisions continue until a considerable amount of diaphragm tissue is 

 formed (Fig. 3). 



The point where each incipient diaphragm meets the outer surface of the 

 meristem marks the base of a whorl of leaf primordia. While these are still 



mm 



Fig. 1. Section through the extreme tip of the growing shoot of Equisctum liycinalc, 

 showing apical cell, surface layer of elongate cells, central mass of irregular cells, and 

 young leaf primordia. 



Fig. 2. An oblique division in one of the surface cells. The smaller one will produce 

 an apical cell of a leaf primordium. 



very small there begins to be differentiated within the base of each a series of 

 provascular strands from which the circle of vascular bundles will later 

 develop (Fig. 3). Each of these strands arises by a series of longitudinal 

 divisions (thus at right angles to those in the diaphragm) in a cell row near 

 the surface of the meristem. These divisions, like those in provascular tissue 

 generally, run parallel to the long axis of the cell. 



In the subepidermal layer of the meristem, along the future ridge of the 

 axis, where the photosynthetic tissue will later develop, the method of division 

 is still different. Here the anticlinal longitudinal walls in a given cell are in 



