FRUIT STRUCTURE 199 



upon close examination they will be found to consist 

 of a pericarp within which is the seed. 



Exocarp Tissues. — As seen in transverse section, 

 the exocarp in" this type of fruits consists of a 

 single layer of colored, rather thick-walled, irregularly 

 elongated cells (Plate. 65). These cells may possess 

 striated walls, rarely show stomata, and may give 

 rise to papillae or unicellular trichomes. On surface 

 view the exocarp appears to be composed of colored 

 polygonal or irregular cells, and is usually adherent 

 to the tissues of the mesocarp. Surface views are 

 usually obtained in powdered materials although an 

 occasional transverse fragment may be apparent. 



Mesocarp Tissues. — The mesocarp layer consists 

 of rather thick-walled parenchyma cells, oil ducts and 

 fibro- vascular elements (Plate 65). This layer varies 

 in thickness, owing to the presence of ribs or elevations 

 which are apparent on the outer surface of the fruit. 

 The number of ribs is fairly constant for each species 

 of umbelliferous fruits. The oil ducts are called 

 vittae and are continuous structures, although they 

 may show markings indicating the junctions of the 

 cells composing them. They extend through the entire 

 length of the fruit. The number of vittae is con- 

 stant for a given fruit, and each duct is surrounded 

 by a layer of thickened parenchyma cells. The 

 fibro-vascular bundles consist of small numbers of 

 short, thick-walled fibers in conjunction with annular 

 or spiral vessels. In powdered materials the par- 

 enchyma appears as a mass of thick, white-walled 

 cells, either separate from or combined with frag- 

 ments of the vittae. The vittae are always seen 

 on longitudinal or surface view, and appear as broad, 



