APIUM GRAVEOLENS 



475 



The narrow layer of procambial cells which finally separates the 

 primary xylem and phloem is the region which may form the 

 cambial-like zone referred to above. 



During elongation, only protoxylem and protophloem elements 

 are matured; but when the petiole ceases to elongate, the pro- 

 cambium gives rise to metaxylem and metaphloem elements. In 

 most bundles, this completes the maturation, since little or no 

 secondary tissue is present in the petioles. 



Ontogeny of the Xylem. — The xylem tissue of the bundle 

 consists only of vessels and parenchyma, no fibers being formed. 



(I b c de f g hi j k I 



Fig. 143. Median longisection through bundle and adjacent tissue of petiole: a, pith; 

 b, protoxylem, including spiral vessels and parenchyma; c, metaxylem; d, pitted secondary 

 xylem vessel ; e, cambium ; /, phloem ; g, bundle sheath ; h, cortical parenchyma ; i, cortical 

 oil duct with epithelial cells ; j, collenchyma ; k, thick-walled parenchymatous cells ; /, 

 epidermis with cuticle. 



The first-formed protoxylem elements are small in diameter, and 

 the majority of them have secondary thickenings of an open spiral 

 type, although infrequently there may be a limited production of 

 vessels with annular bands. As the bundle elongates, the spirals 

 become more and more stretched and some of the protoxylem 

 elements are finally obliterated. The progressively larger meta- 

 xylem elements are scalariform and reticulate. There is a large 

 amount of parenchyma adjacent to the protoxylem, and a limited 

 number of parenchymatous cells surround the metaxylem vessels. 

 (Fig. X43.) 



In Esau's (8) study of vessel development, it is pointed out that 

 the vessel segments expand very rapidly; and, in so doing, affect 

 the spatial relationships of adjacent cells. These are not only 

 flattened and distorted; but, in some cases, actual tearing of tissue 

 may occur so that "new contacts are made by the expanding 



