Differentiation 187 



initial can be traced to a cell of the subepidermal layer. Analysis of 

 differentiation in terms of cell lineage can often be carried further. 



The development of larger organs involves more than a single cell 

 lineage. It may be studied in the differentiation of lateral organs in the 

 apical regions of both root and shoot. From the root there grow only 

 lateral roots, which arise in the pericycle and push out through the cortex. 

 The shoot meristem, however, is more complex ( p. 89 ) . At the base of the 

 terminal dome of cells arises a series of minute protuberances, the early 

 leaf primordia, arranged in a precise order. 



The cause of the differentiation of these primordia from the rest of the 

 apical meristem is not known. Schiiepp ( 1952 ) suggested that, since cell 

 division in the outer layer of the meristem is always anticlinal but in the 

 tissue below may be in various directions, this surface layer will expand 

 more rapidly than the surface of the underlying tissues and will thus 

 tend to buckle or pucker, starting the formation of primordia. This would 

 not explain the very regular pattern in which these arise, however, and 

 it can also be shown that the initial bulge results from division in a group 

 of cells just beneath the surface layer. Snow and Snow (1947) have sub- 

 mitted this theory to experimental test by making shallow incisions at 

 the surface of the meristem. Instead of closing up, as they would do if 

 the outer layers were under pressure, these gaps open, indicating that this 

 region is actually under tension. 



The fate of a small lateral primordium may not always be to grow into 

 a leaf. Wardlaw and his students (p. 71) have performed various experi- 

 ments on the meristems of ferns in which, by deep cuts, they were able to 

 isolate from the apex a young primordium or a region that was about to 

 develop into a primordium. In most cases this structure, instead of form- 

 ing a dorsiventral leaf, developed into a radially symmetrical bud-like or- 

 gan which, in culture, was capable of growing into a whole plant. Factors 

 in the surrounding meristematic tissue evidently help determine into what 

 sort of structure a given primordium will differentiate. 



The growth of the leaf primordium into a mature leaf has been studied 

 by many workers (p. 90, and Foster, 1936). In general, the upper and 

 lower epidermis is continuous with the outer layer of the meristem, and 

 what will later form the palisade and spongy layers is continuous with 

 the subepidermal layer. The veins usually arise from a layer just below 

 this. The differentiation of the leaf of tobacco has been described by 

 Avery (1933) and of Linum by Girolami (1954; Fig. 8-2). Foster (1952) 

 has reviewed the development of foliar venation. The mode of develop- 

 ment and differentiation in certain leaves of unusual shape, as in 

 Podophyllum and Sarracenia, is described by Roth ( 1957 ) . The growth 

 of a fern frond, at least for some time, takes place by the activity of an 

 apical cell (Steeves and Briggs, and Briggs and Steeves, 1958). 



