390 Boodle . — Anatomy of the Schizaeaceae. 
and the cells marked with crosses are endodermal cells. The 
first lateral root-trace joins the stele, and the central paren- 
chyma and sieve-tubes keep changing their position slightly 
so as to be again connected with the outer parenchyma and 
again shut in. The third leaf-trace passes out, the parenchyma 
remains enclosed, and the second root-trace joins the stele. 
The central soft-walled tissue increases and the xylem opens. 
Fig. 26 illustrates this stage. The four elements marked with 
crosses are almost certainly sieve-tubes, the other elements 
immersed in the xylem being parenchyma. Thus the central 
tissue at this stage is to be regarded as phloem. The fourth 
leaf-trace is then given off, leaving the xylem open, and the 
third root-trace is attached. The parenchyma then tends to 
form a more rounded mass with the sieve-tubes in the middle 
of it, and they here have a connexion with the external sieve- 
tubes. After the exit of the sixth or seventh leaf-trace the 
xylem closes and a parenchyma-cell appears in the middle of 
the central group of sieve-tubes. Then several other paren- 
chyma-cells are formed adjacent to it, so that the stage shown 
in Fig. 27 is reached. This is where the eighth or ninth leaf- 
trace is preparing to pass out, and in this figure x' . and pH . 
are the xylem and inner phloem which will pass out in the 
leaf-trace. After this one or two endodermal cells appear in 
the central parenchyma, and a little higher up are connected 
(the xylem being now open) by a single row of endodermal 
cells with the outer endodermis. This is shown diagrammati- 
cally in Fig. 28. The connexion is almost at once broken 
again ; one or two endodermal cells remain in the central 
parenchyma, increase in number, and a parenchyma cell is 
formed in the middle of the endodermal group. Fig. 29 
is a diagram of this stage, and Fig. 30 is a drawing of part 
of the section showing the ring of endodermis and the adjacent 
tissues. This endodermal ring or group, at a later leaf-gap, 
becomes connected with the outer endodermis by a double 
row of endodermal cells so that the horseshoe-shaped stele 
of Fig. 31 is formed. The incurved part of the endodermis 
(e.) surrounds a projection of parenchymatous tissue resembling 
