THE YOUNG SPOROPHYTE 



63 



make a nearly continuous crescent-shaped mass of procambium. Fig. 44, A, B, shows 

 the section of the central stele; and the space between the horns of the crescent, i. e., 

 the space between the third and fifth leaf traces, constitutes the so-called "foliar 

 gap" which faces the second leaf trace. The central cells of the young stele have 

 already begun to form cambium, which in the older stem constitutes such a char- 

 acteristic feature of the stele. 



Before any tracheary tissue is formed in the young stele, a few thick-walled 

 cells can be seen in the outer part of the portion of the stele corresponding to the 

 third leaf trace. These are protophloem cells, and similar ones are developed later 

 in the younger parts of the stele. The first tracheary tissue appears in the region 

 of the steel section belonging to the third leaf trace. In the section shown (fig. 44, C) 



Fig. 44. 

 ABC Theyoungvascularbundlesof sections E,F,G, of the series shown in fig. 43. X150. 

 D. Section of a young sporophyte of Botrychium virglnimum, showing junction of a leaf trace with 

 stele of axis. X75. 



there are three groups of these primary tracheids, separated by a considerable interval 

 and placed at the extreme inner limit of the young stele. In the older portions of 

 the bundle the number of these increases and similar ones make their appearance 

 in the next younger section of the stele, that belonging to the fourth leaf. Last of 

 all these primary tracheids are formed in the younger region of the stele and new 

 ones arise between these first-formed ones, so that there is developed a nearly 

 complete circle of tracheids marking the inner boundary of the tubular stele. 



The second leaf trace approaches nearer and nearer the two horns of the 

 crescentic stele section as it is followed downward, and finally joins it, filling up 

 the gap between the regions belonging to the third and fifth leaf traces. Thus, for a 



