APIUM GRAVEOLENS 473 



periclinal, and later anticlinal divisions result in the formation of 

 a large number of much elongated cells with small diameters. 

 The strand continues to increase in size through repeated divisions 

 of the cells of the strand itself, and through the addition of pe- 

 ripheral cells derived from the adjacent fundamental parenchyma. 

 The divisions occur so rapidly in the early ontogeny of the col- 

 lenchyma that the resultant cells are much smaller than those of 

 the surrounding tissue. 



Cell enlargement does not begin until cell division is slowed 

 down, at which time the deposition of the characteristic wall 

 thickenings is initiated. In their meristematic condition, the 

 young collenchymatous cells are longer than wide with transverse 

 or slightly oblique end walls which become more tapering as 

 growth proceeds. In later stages of development, thin transverse 

 walls may be laid down. The mature cells retain their protoplasts 

 and their potentiality for growth and enlargement. The walls are 

 chiefly cellulose with a high percentage of water, but they also 

 contain pectic substances which are more abundant near the middle 

 lamella. 



Anderson (x) has found that the cellulose and pectic substances 

 of the collenchyma occur in alternate layers and has suggested 

 that this alternation may help to explain the fact that contraction 

 in the transverse plane is much more pronounced than in the 

 longitudinal one. He points out that this also may account for 

 the high percentage of water found in the walls of the collenchyma, 

 which greatly exceeds that found in those of lignified cells. 



Ontogeny of the Bundle. — The vascular bundles are endarch 

 and collateral. At maturity each consists of a xylem region which 

 is oriented toward the adaxial surface and a phloem zone which 

 partially surrounds the abaxial face of the xylem so that the bundle 

 may be half-amphicribral. The walls of the outermost phloem 

 cells thicken to form a strengthening cap on the abaxial face of the 

 bundle. A weak cambium may be formed, but the amount of 

 secondary tissues cut off by it is relatively small so that the major 

 portion of the vascular tissue can be regarded as primary. 



In the ontogeny of the bundle, the procambial strand gives rise 

 to protoxylem and protophloem cells which are located rather 

 near to each other; but, according to Esau, 



"continued division of procambial cells in the center of the bundle 

 increases the distance between them. The procambial cells formed 



