ONTOGENY OF STELE IN VASCULAR PLANTS 105 



differentiation of the jiifJi. This sets apart, near the base of the 

 shoot apex, a peripheral zone of tissue which shows little or no 

 evidence of specific differentiation. The next step, as one pro- 

 ceeds away from the summit of the apex, is concerned with the 

 effects produced by the developing leaf primordia. These struc- 

 tures early assume a dorsiventral character and their abaxial 

 regions begin early to develop as parenchymatous tissue. As a 

 result, at least in many dicotyledons, trans-sections below the 

 base of the shoot apex, show a more or less distinct "ring" of 

 embryonic tissue demarcated on the outside by the united bases 

 of the leaves and on the inside by the developing pith. This tis- 

 sue zone has been designated by Helm as the "meristem ring" 

 and by Louis as the "prodesmogen." From it there is produced 

 the provascular or procambial cells which ultimately give rise 

 to the primary vascular system of the stem. 



Divergent ideas are held as to the way in which procambium 

 arises from the meristem ring or prodesmogen. According to one 

 view, the procambium arises as isolated strands in the bases of 

 the young leaf primordia, from which points its further differ- 

 entiation proceeds acropetally towards each leaf apex and basi- 

 petally towards the prodesmogen tissue in the axis. However, 

 a number of recent studies (e.g., Boke, 1940, 1941) suggest that 

 this may be an erroneous interpretation and that the develop- 

 ment of provascular tissue in the region of the shoot apex may be 

 exclusively acropetal. 



One of the important aspects of recent studies has been to 

 emphasize the interrelationship which exists between stem and 

 leaf in the building up of the stele. This is shown by the fact 

 that in conifers and many dicotyledons, the first vascular bundles 

 in the young siphonostele are leaf traces or so-called "common 

 bundles. ' ' The interfascicular areas in the ' ' meristem ring ' ' may 

 either produce additional procambial tissue from which addi- 

 tional primary xylem and phloem arise, or as in dictyosteles, 

 progress towards the formation of parenchyma. In the latter 

 case, the interfascicular strips either mature as typical medullary 

 rays or give rise to an interfascicular camhium. 



The question of the differentiation of primary phloem and 

 primary xylem in leaf traces has apparently received only meagre 



