THE AXGIOSPER.MAE : LEAVES gyy 



observed that the pinnae arise on the side of the rudimentary rachis as 

 hemispherical cushions, replacing the continuous marginal ridges of meri- 

 stem which are characteristic of the simple leaf blade. These separate 

 rudiments usually arise in basipetal succession, but cases occur of the opposite 

 succession, and also of double succession from the middle in both directions 

 simultaneously. 



The rudimentary lamina consists of five to eight layers of cells and it 

 grows by a marginal meristem. The outer cells of this margin seem to divide 

 chiefly in the anticlinal direction, producing the upper and lower epidermis 

 respectively. By periclinal division, however, they produce a line of sub- 

 marginal initials and it is from these latter that the internal tissues of the leaf 

 are produced (Fig. 968). The course of differentiation may follow various 



Fig. 968. — Nicotiana tabaciim. Transverse 

 section of the edge of a young leaf lamina 

 showing genesis of the tissues from marginal 

 and submarginal initial cells respectively. 

 {After Adiiance Foster.) 



sequences, which differ in detail, but in most cases there seem to be three 

 primitive cell-layers formed. The upper or adaxial layer produces the palisade, 

 the middle layer forms the veins and part of the spongy layer, while the lower 

 or abaxial layer develops into the lower part of the spongy parenchyma 

 (Fig. 969). 



Marginal growth ceases as soon as the main outline of the mature leaf 

 has been formed and is succeeded by a protracted period of generalized 

 surface growth, characterized by the predominantly anticlinal division of 

 the cells. The result of this is that the number of layers laid down bv the 

 marginal meristem remains practically constant throughout the later develop- 

 ment, the only important exception being the formation, by periclinal 

 divisions in the middle layer, of procambium strands from which the veins 

 are developed. The simultaneous longitudinal extension of the veins and 

 superficial growth of the other parts of the lamina, lead to adjustments of 

 position, which are responsible for the characteristic ptyxis or folding of the 

 young leaf within the bud. The final shape of the mature leaf is determined 

 by local variation of growth rates in difi'erent parts of the lamina, which may 

 be due to variation in the distribution of auxins in the tissues. 



32 



