64 



Growth 



suffer displacement, transversely and longitudinally, if growth is more 

 rapid in some regions and directions than in others. In such broad 

 apices as those of Microcycas, the normal pattern has been greatly 

 modified and the tip flares out in a fan-like fashion (Fig. 4-10). What 

 these facts mean morphogenetically we do not know, but they show that 

 the growing apex has a pattern of organization which develops in a pre- 

 cise fashion. 



The shoot meristem is by no means homogeneous or structureless in 

 other particulars. In recent years many students have come to recognize 

 a rather uniform series of zones within it, distinguished not primarily by 

 layers or planes of cell division but by differences in the character of 

 their cells. A general survey of zonation in vascular plants has been 

 made by Popham ( 1951 ) , who, from his own work and a long series of 

 published descriptions of meristems, has grouped them into seven classes. 



Fig. 4-11. Diagram of zonation in the shoot apex of Chrysanthemum morifolium. 1, 

 mantle layer; 2, central mother-cell zone; 3, zone of cambium-like cells; 4, rib meri- 

 stem; 5, peripheral zone. (From Popham and Chun.) 



In the vascular cryptogams there are one or more apical cells or a sur- 

 face meristem, with tissues below sometimes differentiating into a cen- 

 tral and a peripheral meristem. Among seed plants, four or five zones 

 can be seen (Fig. 4-11). These are a surface zone, or mantle, including 

 two to several cell layers and corresponding roughly to the tunica; a 

 zone of subapical mother cells, irregular in shape, often rather highly 

 vacuolate and dividing less rapidly than the surrounding ones; a central 

 zone giving rise to the rib meristem and pith; and a peripheral zone just 

 outside this, producing cortex and procambial tissue. In some plants, 

 just below the mother-cell zone there is a somewhat cup-shaped arc of 

 cells stretching across the axis, the cambium-like zone. Popham and 

 Chan ( 1950 ) and Popham ( 1958 ) have described a typical case of this 

 last type. The particular functions of these zones are not well understood 



