0. A. Derby — Stem Structure of Tietea Singularis. 257 



level of plane 5 ; two groups of F strands of which f2' had 

 commenced to be differentiated from the mother central strands 

 between the levels 8 and 7 (see fig. 3), while fY had just begun 

 the process of differentiation, and a number of central strands 

 that, with individual changes unessential to the present study, 

 had come up from plane 14. 



In fig. 2B the Fl and P2 groups of strands with their suc- 

 cessors are shown in their entirety together with the right por- 

 tion of the PI group, of which only a part is preserved in the 

 specimen. The first of these groups appears in its adult condi- 

 tion on the three lower planes, but is lacking on those above. 

 Its successor {fl'-FV) had evidently just begun to form below 

 plane 14 on which it appears as a single row of small strands 

 lying behind the slightly older formative stages of a new set of 

 P clusters (pi" + p2 ff ). Before plane 10 was reached the nas- 

 cent f strands had develojDed into two long rows, and the p 

 ones had commenced to separate into a right and left cluster 

 that at plane 8 had reached, or nearly reached, their adult con- 

 dition accompanied by the appearance behind them of scleren- 

 chymous bands setting them off from the still immature F group 

 of strands. At the level of plane 7 the latter had pushed far 

 to the sides the sclerenchymous bands with the accompanying 

 P clusters, and was nearly prepared for its lateral emergence 

 that was effected between this plane and the one above. In 

 the process of emergence the formation of stem rind in front 

 of the organ ceased, probably from the overlapping of the lat- 

 ter that had previously risen erect from the vegetative disc, 

 and a sclerenchymous band formed behind the group, which 

 uniting with the incurved lateral bands reestablished the con- 

 tinuity of the stem covering. Before the level of plane 6 was 

 reached a new group of P strands (pi"' -\-p2 n/ ) had commenced 

 toform,but its accompanying F group (fl") had not yet appeared 

 so that it only presents itself on plane 5. The upper plane (2) 

 is essentially a reproduction of the lower one (14) with some 

 of the central strands added, so that by imagining the preserved 

 portion of the Fl' group prolonged by emending with it that 

 of the Fl' one, we have the entire basal portion of an F organ 

 from its first inception up to the point where its dead free por- 

 tion fell away from the living plant, leaving a persistent stalk 

 base (compare Psaronius hrasiliensis, this Journal, Nov. 

 1913). The older P clusters (Pi, P2, P2') shown in this figure 

 present no features that have not been sufficiently discussed in 

 the study of fig. 2 A. 



In fig. 3 the peripheral members of the central group of 

 strands that appear on plane 14 just behind the f2 group 

 have been plotted up to plane 2. As will be seen by reference 

 to fig. 1, these do not present on section 14 any features that 



