APIUM GRAVEOLENS 479 



thickened. These cells retain their protoplasts; and, as they 

 mature, the end walls, which are at first transverse or slightly- 

 oblique, become more tapering. The wall thickening consists of 

 cellulose, and its distribution resembles that of collenchymatous 

 cells. (Fig. 2.43.) The obliteration of sieve tubes and companion 

 cells is the result of active growth of the phloem parenchyma, and 

 the same displacement of cells may occur which has been noted 

 in the growth of the xylem vessels. 



Oil Ducts. — Oil ducts occur in both the protophloem and 

 metaphloem, and their formation in this region, as well as in the 

 pith and cortex, is schizogenous. They are commonly formed by 

 the development of an intercellular space at a point where three 

 cells are in contact. The schizogenous splitting of the adjacent 

 cell walls results in the formation of the duct, which is at first 

 bounded by the three original cells but successive divisions of 

 these cells may produce several epithelial cells. In the phloem, 

 the schizogenous formation of intercellular spaces may occur so 

 rapidly that additional cells are brought into contact with the 

 duct by the separation of pairs of cells. The epithelial cells are 

 densely cytoplasmic with large nuclei, and secrete an ethereal oil 

 which accumulates in the duct. The duct is a continuous canal, 

 and the epithelial cells surrounding it are arranged end to end in 

 rows. (Fig. X43, L) 



Mechanical Tissue of the Petiole. — The only tissues which 

 contribute to the strength, and possible toughness or stringiness 

 of the petiole, are the cells of the collenchymatous strands, the 

 phloem elements which constitute the bundle cap, and the lignified 

 elements of the xylem. 



Esau (7) determined the relative breaking load of strands of 

 these three tissues. In all cases, it was found that the collenchym- 

 atous cells were much tougher than those of either the xylem or 

 the bundle cap; and that they also exceeded the combined strength 

 of the last two tissues. This was not because of the size of the 

 strands of collenchyma, for in many cases they were smaller than 

 the entire vascular bundle, in some instances being thinner than 

 the bundle caps alone. Differences in the strength of collenchyma 

 were found in comparing varieties of celery; and tests indicated 

 that the collenchymatous strands of Tall Golden Self-Blanching 

 were weaker than those of Golden Plume, while some of the strands 

 from an old leaf of the Utah variety were most tenacious. Varia- 



