MARGINAL AND APICAL CELLS 



7!) 



admirably illustrates this mode of growth. If such a cell-plate be 

 supposed to rotate around either a transverse or a longitudinal axis, a 

 spherical cell-body will eventuate ; here the superficial cells effect a 

 uniform expansion of the cell-body on all sides, and in their totality 

 constitute the primordial meristem. In this case also, of course, the 

 final result is the production of cellular filaments radiating outwards 

 in every direction. 



A more definite localisation of the primordial meristem prevails 

 where the cell-plate or cell-mass grows most rapidly in one particular 

 direction, where, in other words, a distinction can be drawn between 



Fi<;. o. 

 Melobcsia LcjolUii (Florideae). After Rosanoff and Sachs (from Sachs, Lectures). 



longitudinal and transverse growth. In this event the cells have 

 unequal powers of growth at different points in the margin. Those 

 which occupy the anterior margin grow and divide most rapidly : they 

 constitute the apical region, and are responsible for the apical growth 

 of the elongating organ. In some cases such an apical region appears 

 to consist of a number of divergent curved cellular filaments, ter- 

 minating at the apical surface in marginal cells which are equivalent 

 among one another (Fig. 1 0). It is more usual, however, for the cell-walls 

 in an apical region to be arranged in such a manner that a single one 

 among the marginal cells of the growing-point acts as the definite 

 initial cell, from which all the remaining cells of the primordial meri- 

 stem are genetically derivable. This primordial mother-cell occupies 

 the apex of the growing-point, and is consequently termed the apical 

 cell. In other cases, however, the arrangement of the cell-walls in the 

 primordial meristem renders it necessary to assume the existence of 



