PLANT ORGANS AND ORGANISMS 
167 
Histology of a Typical Mericarp, Fceniculum. — This five-angled 
fruit, in transverse section, shows a concave commissural and convex 
dorsal surface. Passing from the surfaces toward the center we note: 
1. Epicarp , or outer covering tissue, composed of colorless epider- 
mal cells and small stomata. The epidermal cells in cross-section 
appear rectangular, while in surface view they are both polygonal and 
rectangular. 
2. Mesocarp, of several layers of thin-walled colorless isodia- 
metric cells, beneath which are two to several additional layers of 
thicker-walled cells with brownish walls. Through the angles or 
Fig. 69. — Photomicrograph of a transverse section of a mericarp of Foeni- 
culum vulgar e, showing epicarp (A), mesocarp (B), endocarp ( F ), vitta (C), endo- 
sperm of seed (D), carpophore (G) and fibro-vascular bundle in primary rib (£). 
(Highly magnified). 
rib portions of the mesocarp extend the fibro-vascular bundles. 
Between each fibro-vascular bundle and the tip of each rib will be 
found a zone of collenchyma cells. In the mesocarp between each 
two ribs on the dorsal side occurs a single vitta or oil tube which is 
lined with a layer of brownish polygonal cells. These vittas contain 
the official oil of fennel. Two vittae generally occur in the meso- 
carp of the commissural side although four are reported to have 
been found in this region of some fennel fruits. 
3. Endocarp , a narrow layer of cells, transversely elongated, except 
over the regions of bundles where they may be seen elongated in 
several directions. 
