THE STELE 103 



condition is formed of thin-walled parenchyma tissue. Often, 

 however, the cortex is more complex histologically and exhibits 

 an outer, subepidermal area of collenchyma (which occurs as a 

 continuous c^dinder or as separate strands) and an inner region 

 of parenchyma. Other cell types may also appear in the cortex, 

 particularly sclereides, fibers and secretory cells. 



{c) The stele. This region of the stem includes the primary 

 vascular tissues as well as variable amounts of parenchyma. 



Since the original formulation of the "Stelar Theory" by 

 Van Tieghem and Douliot in 1886. comparative studies on the 

 stem have largely centered upon the organization aiul phylogeny 

 of the stele, particularly with reference to the distribution of 

 phloem and xylem and the morphological nature of the pith. It 

 is now rather generally held that two principal types of steles 

 occur in the sporophyte of vascular plants, viz.: (1) the proto- 

 stele, which consists of a central core of xylem ensheathed by 

 phloem, and (2) the sipJionostele which is characterized by the 

 presence, internal to the protoxylem, of a central mass of paren- 

 chyma known as the pith. The evidence from comparative anat- 

 omy, including the facts of paleobotany, supports the idea that 

 the protostele is the primitive type. This type of stele is found 

 in both the stem as well as the root of many lower tracheophytes 

 but is restricted to the root of seed plants. The order of matura- 

 tion of the tracheary elements of the xylem in a protostele is 

 centripetal and primary xylem of this kind is termed exarch. 

 In contrast, the protoxylem in a siphonostele is situated at the 

 outer edge of the pith and the order of maturation of the xylem 

 is centrifngal. Primary xylem in the stems of seed plants is thus 

 endarch. A discussion of the involved controversy as to the way 

 in which the siphonostele may have originated from the proto- 

 stele is beyond the scope of this book and the student is referred 

 to the excellent resumes given by Eames and MacDaniels (pp. 

 112-114 and 337-340) and Smith (1938, pp. 124-131). In the 

 writer's opinion, however, the evidence from ontogeny appears 

 to favor the idea that at least in seed plants the pith region is 

 morphologically a part of the stele and represents the paren- 

 ehymatization of potential vascular tissue. As seen in trans- 

 section, the vascular tissue of the siphonostele appears either as 



