Manchester Memoirs, Vol. lix. (191 5), No. 8. 43 



secondary sieve-tubes similar to those of the primary 

 phlcem. When, however, the first anomalous secondary 

 tissue to be laid down develops as xylem, there is no 

 possibility of confusing it with normal secondary xylem, 

 for it lies outside the primary phlcem, which is continuous 

 into the leaf traces. Indeed, the primary phloem is 

 brought out more clearly in such specimens. An example 

 is shown from a transverse section of the stele of Isoetes 

 setacea in Photo 19, and its inclusion with the illustrations 

 of the structure of /. lacnstris will serve not only to show 

 this type of secondary development but the similarity in 

 construction of the steles of these two species. This 

 essential agreement, as stated in the beginning of the 

 paper, holds for all the species compared. 



If we now sum up the result of the analysis of the 

 stele and surrounding tissues of Isoetes in the light of the 

 apical development, we find that the following regions 

 can be distinguished from within outwards : — 



a. Central column of primary xylem ; this is a strictly 



cauline region of the stele. 



b. Peripheral zone of xylem. 



//. Bases of xylem of leaf-traces, joining on to 

 the central column of xylem (a). 



U 1 . Radially seriated xylem between the entering 

 leaf traces. 



c. Parenchymatous xylem sheath, continuous with 



similar region in the leaf-trace. 



d. Primary phlcem, continuous with phlcem of leaf- 



trace. 



e. Secondary prismatic tissue (tracheides, sieve-tubes 



or parenchyma). 



f. Meristem of secondary prismatic tissue, originally 



established just outside the primary phloem. 



g. Cortical tissue. 



