464 THE STRUCTURE OF ECONOMIC PLANTS 



leaves and their subsequent development results in the formation 

 of the much branched shoot of the second year. 



Anatomy of the Floral Axis. — The mature floral axis is 

 strongly ridged and fluted and may or may not become hollow^ at 

 maturity. The outer walls of the epidermis are thickened and 

 covered with a somewhat roughened cuticle. Stomata occur in 

 the portions of the epidermis overlying the chlorenchyma, and 

 Nestel (16) has indicated their frequency at about 70 per square 

 millimeter. The zone of chlorenchyma which lies within the 

 epidermis, except at the angles, is four or five layers in width, and 

 the round or elliptical cells are slightly longer than broad. The 

 non-chlorophyllose parenchyma of the cortex consists of large 

 cells that are more loosely organized than those of the chloren- 

 chyma. (Fig. 137. ) They occupy the region between the chlo- 

 renchyma and vascular cylinder, also separating the collenchyma 

 from the large bundles located at the angles of the stem. 



The strands of collenchyma constitute the chief mechanical 

 tissue of the axis and form the ridges, being separated from the 

 epidermis by one or two layers of parenchyma which later become 

 thick-walled. These cells are larger in transection than the 

 coUenchymatous elements; and, in longisection, can be dis- 

 tinguished from them since they are only two to three times as 

 long as broad with transverse end walls, while the coUenchymatous 

 cells are much elongated and sharply pointed or oblique. Addi- 

 tional strands of collenchyma also may be located in the grooves 

 between the ridges, and these are separated from the epidermis by 

 chlorenchyma rather than by thick-walled parenchyma. The 

 coUenchymatous strands at the ridges are oval or kidney-shaped 

 in transection, while those in the intervals are more or less circular. 

 Centrad to each strand is an oil gland or duct, and similar ones 

 commonly occur outside the phloem of each vascular bundle. 



The vascular bundles are collateral, the larger ones lying on 

 radii which pass through the angles of the stem. Nestel, in a 

 comparative study of the stems of the Umbelliferae, points out 

 that celery belongs to the type in which there is no interfascicular 

 cambium. Because of this, the adjacent bundles are distinct from 

 one another, except that the medullary rays are occluded on the 

 xylem side of the ring. This results from the differentiation of 

 the ray parenchyma to form connective or conjunctive tissue by 

 lignification and wall thickening as the stem matures. The 



