104 THE STEM 



a continuous cylinder, broken only by the short leaf gaps in nodal 

 regions, or as a "ring" of vascular bundles. A stele of the latter 

 type should be visualized as a tubular network, the "meshes" 

 representing the long vertical leaf gaps or parench^-matous rays. 

 This kind of stele is termed a dictyostele and its component bun- 

 dles are either collateral or hicollateral. In gymnosperms and 

 dicotyledons, the siphonostele is specifically designated as ecto- 

 pliloic if there is only an external area of phloem, or as ainphi- 

 phloic if both internal as well as external phloem occurs. This 

 latter t^'pe of siphonostele is restricted to certain families in the 

 dicotyledons and also appears in the stems of some ferns. The 

 nature of the houndary between stele and cortex has produced 

 much discussion among anatomists. A common view is that the 

 cndodermis represents the innermost layer of the cortex and that 

 hence all tissues internal to it, including the peincycle, vascidar 

 tissues and pith, constitute the stele. Such a demarcation is 

 practical in roots which typically develop a well-defined cndo- 

 dermis, and the evidence of ontogeny indicates that at least in 

 some plants, the endodermis is morphologically a part of the cor- 

 tex (cf. Esau, 1941). In the stems of seed plants an endodermis 

 or its equivalent (i.e., the so-called ^'starch sheath'^) may be 

 present. Under such circumstances it should be clear that until 

 further ontogenetic evidence becomes available, only an approxi- 

 mate and somewhat arbitrary demarcation can be made in many 

 stems between the innermost region of the cortex and the adja- 

 cent pericycle. 



2. The ontogeny of the srele in vascular plants. Within recent 

 years there lias been a inai-ked revival of interest in the origin and 

 differentiation of the ])rimary vascular system. Among the more 

 important contributions may be cited the work of Helm (1932), 

 I>ai'thelmess (1935), Louis (1935), Kaplan (1937), and Gregoire 

 (1938). One of the principal objectives in these investigations 

 has been to determine how the jn-ovaseular tissue or procambium 

 is produced from the tissue of the shoot apex. In view of the 

 involved aspects of this process, it will only be possible to outline 

 certain important steps. 



Ill a iium])er of dicotyledons, the first stage in tlie determina- 

 tion of the position of the provascular tissue consists in the early 



