IPOMOEA BATATUS 



507 



oxalate are abundant in the parenchymatous cells of both the 

 cortex and pith. The endodermal cells are distinct and can be 

 recognized by the relative density of their cell contents. Within 

 the endodermis, there are two or three layers of pericyclic cells 

 and abutting these are small scattered groups of outer phloem con- 

 sisting of narrow thin-walled elements. 



Early in ontogeny, the procambial ring is well defined so that 

 it forms a continuous cylinder in the young stem. The vessels 

 of the primary xylem are spiral or variously reticulated. The 

 inner phloem, like the outer, is comprised of isolated groups of 

 sieve tubes and companion cells which are much smaller in diameter 

 than the adjacent parenchyma of the pith. They are not always in 

 radial alignment with the primary 

 xylem; and, for this reason, do not 

 exhibit a strictly bicollateral ar- 

 rangement such as occurs in the 

 Cucurbitaceae. The parenchyma- 

 tous cells of the pith are large with 

 intercellular spaces, and there are 

 numerous latex canals and crystal 

 inclusions similar to those found 

 in the cortex. 



The Mature Stem. — In the 

 mature stem, the epidermis is some- 

 what thick-walled and CUtinized ^^cesso^y cells :.., accessory cells ; ^ ., 



guard cells. 



with relatively few glandular and 



non-glandular hairs so that the surface is usually glabrous. Len- 

 ticels are numerous and the tabular thin-walled cork cells form- 

 ing them are produced by periclinal divisions of the epidermal cells 

 adjacent to a stomatal opening or by similar divisions of the hypo- 

 dermal layer. In cases where periderm formation has occurred, 

 the cells of the hypodermis are in radial alignment with the adja- 

 cent epidermal cells. Within the hypodermal layer are several 

 rows of compact parenchymatous or collenchymatous cells which 

 form the outer zone of the cortex. The inner zone consists of large 

 parenchymatous cells which undergo considerable tangential 

 elongation and some anticlinal division to compensate for the 

 secondary thickening of the stele. The latex canals persist but are 

 somewhat difficult to recognize because of the increased size of the 

 adjacent cells. The endodermis is well defined, and, in the under- 



FiG. l66. ^, detail of young glandular 

 scale ; B, young non-glandular epidermal 

 hair ; C, stoma with guard cells and 



