SOLANUM TUBEROSUM 517 



Ontogeny of the Stem. — Artschwager (i, x) has investigated 

 the ontogeny and vascular anatomy of the stem and tuber, and the 

 following account is based upon his report. The first differentia- 

 tion in the apical meristem is the demarcation of the dermatogen, 

 a peripheral layer of cells giving rise to the epidermis; and the 

 appearance of the procambial ring that separates the parenchyma- 

 tous tissue of the pith from that of the cortex. The procambial 

 ring IS the forerunner of the stele; and the small, elongated, thin- 

 walled cells which comprise it are filled with a dense cytoplasm. 

 They are readily distinguished from the adjacent parenchymatous 

 cells of the cortex and pith which are less dense, much larger, and 

 more nearly isodiametric. 



As the leaf primordia arising from the growing point of the 

 stem undergo growth and development, the downwardly differen- 

 tiating bundles of their traces first appear in the procambial ring. 

 The first vascular elements to mature are the protoxylem cells 

 located in the median zone of the procambial cylinder. They are 

 long and slender and have annular wall thickenings in which the 

 rings are spaced at a considerable distance from one another. 

 Some of the protophloem cells may differentiate simultaneously 

 with the development of the protoxylem, but they are not structur- 

 ally different from the adjacent procambial cells and cannot be 

 distinguished from them. Shortly after the formation and devel- 

 opment of the protoxylem, the inner phloem is differentiated. 



There is a centrifugal development of the succeeding primary 

 xylem elements so that they are endarch in orientation. The later- 

 formed protoxylem elements are somewhat larger than the initial 

 ones, and their secondary wall thickenings are characterized by 

 a series of close rings or loose spirals. As differentiation pro- 

 ceeds, groups of phloem initials are formed in the outer portion 

 of the procambial zone. These are similar to, but less numerous 

 than, those differentiated earlier in the procambial region adjacent 

 to the pith. The cells of the undifferentiated portion of the 

 procambial ring, between the primary xylem and the outer phloem, 

 appear very regular in arrangement owing to a series of tangential 

 divisions which mark the initiation of cambial activity. This 

 activity does not at first result in the formation of a continuous 

 ring, and at this stage there is a series of discontinuous sectors 

 of fascicular cambium lying on the same radii as the primary xylem 

 and phloem groups. 



