APIUM GRAVEOLENS 471 



"quickly broadens into a shell-like form, and grows vigorously at 

 its apex. Beneath the apex, protuberances arise at the right and left 

 angles in acropetal order; these also grow in the same manner at their 

 apex, and produce again lateral protuberances of the second order; 

 and according to the extent to which the surface of the leaf is developed, 

 these protuberances become lobes of a simple leaf or distinctly separated 

 leaflets." (Fig. 31, A, B, C.) 



Esau (7) has described in detail the histogenesis of the petiole 

 in which there is an early differentiation into three meristematic 

 regions, the protoderm or dermatogen which gives rise to the 

 epidermis, the procambium which produces the vascular strand, 

 and the ground meristem which becomes the fundamental paren- 

 chyma. The cells of the three meristems can be distinguished 

 early in ontogeny by their size, degree of vacuolation, and the 

 character of the cell divisions which they undergo. The cells of 

 the ground meristem are the largest and the first to develop promi- 

 nent vacuoles and intercellular spaces. The small protodermal 

 cells are more dense than those of the ground meristem ; and the 

 procambial cells are the least vacuolate and smallest in transection 

 of the three types, but are longer than the others in their axial 

 dimension. The cells of the protoderm divide only anticlinally, 

 producing the epidermis and stomatal cells. The ground meristem 

 divides in three planes, but the divisions are less frequent than in 

 either of the other histogens, and they compensate in part for this 

 by more rapid cell enlargement. 



The procambial tissue arises in the ground meristem through 

 a series of rapid periclinal longitudinal divisions, and the strands 

 which are the forerunners of the larger bundles are differentiated 

 in this manner earlier than those which form the smaller interven- 

 ing bundles and the adaxial ones. 



According to Ambronn (i) the strands of collenchyma differen- 

 tiate from the procambial tissue, later becoming separated from 

 it through the differentiation of an intervening layer of paren- 

 chyma. This is not in agreement with the work of Esau (7), 

 who finds that the collenchyma and the vascular bundle have 

 independent origins, and that they are clearly set apart from 

 each other. Their separate origin is emphasized by the early 

 differentiation of an oil duct which lies in the parenchyma be- 

 tween the collenchyma strand and the bundle cap outside the 

 phloem. (Fig. x4i.) 



