34 



THE STRUCTURE OF ECONOMIC PLANTS 



Phloem Fibers. — Both primary and secondary phloem fibers 

 occur, which may be non-sclerotic or sclerenchymatous. They 

 are usually arranged in groups or zones, frequently bounding the 

 periphery of the phloem. They also occur as bands which alternate 

 with sieve tubes and companion cells; and, in other cases, they are 

 scattered singly or in small groups throughout the tissue. The 

 thick walls may be somewhat lignified at maturity, differing from 

 those of the xylem fiber chiefly in the presence of simple rather 

 than bordered pits. 



SECRETORY TISSUES 



Glands. — The gland may consist of a single secretory cell, such 

 as the glandular hair in Cannabis and Solanum; or several cells 

 may form a glandular organ or nectary. These cell aggregates 

 differ in their method of formation and in degree of complexity. 

 Frequently, the gland has a central cavity in which the secreted 

 substance is stored. The cavity may be formed schizogenously by 

 the splitting apart of the walls of adjacent cells and the subsequent 

 formation of a duct by the growth and realignment of the bordering 

 cells. Ducts of this type are termed schizpgenous, and a common 

 example occurs in the primary oil ducts of the Umbelliferae (Apium 

 and Pastinaca). (Fig. ii.) In other cases, the formation of the 

 gland or duct may result from the destruction or solution of groups 

 of cells, thus forming a cavity or tube, surrounded by other cells 



Fig. II. A, sector of the pericycle of the parsnip root showing an initial stage in the 

 formation of the oil ducts; B, a later stage in which the oil ducts have been formed; C, a 

 section of the primary phloem showing the phloem oil duct: c pri od, central primary oil duct; 

 en, endodermis; ept, epithelial cells; pel, pericycle; ph, phloem; ph od, phloem oil duct; 

 pri od, primary oil duct. 



