O. A. Derby — Stem Structure of Tietea Singularis. 259 



in part, by the coalescence of smaller strands and one of them 

 came to end between planes 6 and 5, while at about the 

 same level the other two seem to Lave taken on a new lease of 

 life by subdivision, or perhaps more probably by sprouting, 

 and at plane 2 they present substantially the same conditions 

 as at plane 14. As it seemed reasonable to assume that the 

 central strands in general would present essentially the same 

 features as those here represented, two of the most prominent 

 of the more centrally placed ones were plotted separately 

 with the result that the correctness of this assumption was 

 fully verified. 



At plane 14 the four mother strands {/%') present no features 

 to distinguish them from their neighbors of the central 

 group. At plane 12 a new strand had appeared behind 

 the third, counting from the left, of this group of four, 

 which, on being traced upward, proved to be one of the 

 progenitors of a new group of F strands {f%"). A companion 

 to this, situated more to left, had already appeared on 

 plane 11, but owing to its position in the background 

 this set of strands could only be represented near its 

 origin and, in a subdivided condition, on plane 2 where it 

 presents an aspect quite similar to that of its predecessor on 

 plane 11. AYhether these new strands originate from the as 

 yet undifferentiated ones in front of them or from the adjacent 

 central strands behind them, could not be determined, but the 

 latter hypothesis seems to be the most plausible one. Between 

 planes 12 and 10 the two central y'2' strands had united and 

 thrown off a vigorous branch to the left, while the one at 

 the left of the group had given off a branch to the right which 

 persisted to the higher levels after its companion came to an 

 end between planes 7 and 6. At the level of plane 8 fine 

 small strands had appeared in front of the central and right 

 members of the f%' group from which they had evidently 

 sprouted at a point above plane 10. These are the beginnings 

 of a new pair of P organs, which, in order not to obscure 

 the figure, are represented as cut off at this plane, their 

 prolongation being shown in the separate figure at the right. 

 Above plane 8 the/'2 / strands began to multiply by subdivision 

 or sprouting and this process, continued nearly up to the level 

 of plane 2, increased their number about eight-fold. At the 

 latter plane, the group as a whole was nearly ready for its 

 lateral emergence, being in a considerably more advanced 

 stage of development than its predecessor at plane 14, as is 

 shown by its greater dimensions and the wider space between 

 the inner ends of the internal sclerenchymous bands in front 

 of it. These bands had begun to form a little below the 

 level of plane 5, so that it was probably about half way 



