282 TEXTBOOK OF BOTANY 



cium oxalate crystals. Various canals or ducts, such as resin ducts, essen- 

 tial oil ducts, and latex tubes, may also be found in the cortex of some 

 stems. No one of these features, however, is limited to the cortex or even 

 to the stem of a plant. 



Latex, the milky juice found in many species of plants, is collected in 

 large quantities from a few kinds of plants and converted to rubber. 

 Latex tubes, which originate from cells in the embryo in some species, 

 grow through the parenchyma tissues of the plant as if they were para- 

 sites. In other species they are formed by the disintegration of scattered 

 parenchyma cells. Resin ducts are surrounded by special kinds of cells. 



The pericycle. As noted earlier, the pericycle is the peripheral cylinder 

 of the stele ( vascular cylinder ) and in cross section appears as a ring of 

 cells. It varies from one to several cells in thickness. When it is composed 

 entirely of parenchyma it may be difficult to recognize. In fact, in some 

 stems cortical parenchyma appears to extend all the way from the epi- 

 dermis to the phloem; that is, there is no visible differentiation of either 

 endodermis or pericycle. When sclerenchyma ( pericycle fibers ) is pres- 

 ent, the pericycle is easily recognized. These fibers may occur as con- 

 tinuous cylinders or as separate strands, and because of their location 

 they often appear to be the outer part of a vascular bundle. Some of the 

 important fibers of commerce are pericycle fibers — for example, flax and 

 hemp. Those of flax are mainly cellulose, and are less lignified than hemp 

 fibers. 



The pericycle is important in one other way. It is the region in which 

 the primordia of lateral roots, and frequently those of adventitious stems, 

 develop. It is from the pericycle that roots usually develop on stem cut- 

 tings. In some species, such as willow and cottonwood, root primordia 

 develop in twigs on the tree and remain dormant unless the twig is ex- 

 posed to conditions favorable to their further development. 



The phloem. The phloem is a complex tissue in which several kinds 

 of cells may usually be recognized with the aid of the microscope. The 

 chief food-conducting cells are the sieve tubes, which are generally 

 elongate, short-lived, and interconnected through perforated diaphragms 

 called sieve plates (Fig. 104). Associated with the sieve tubes, perhaps 

 in all the vascular plants except ferns and gymnosperms, are long-lived, 

 slender parenchyma cells called companion cells. Various other paren- 

 chyma cells, not associated directly with the sieve tubes, may also be 

 present. All these cells together constitute the part of the phloem in 



