50 DR. D. H. SCOTT ON A NEW TYPE OF MEDULLOSEZ 
Course of the Meristeles. 
We will begin by following the course of the meristele designated a, as shown in the 
series of transverse sections, beginning with the lowest. The sections will be referred 
to simply by the Roman numerals indicating their position in the series (see table 
on pp. 46-47). 
In the lowest section (I.) the future meristele 2 is still completely united to the stele, 
and is represented by a conspicuous promontory about 8 mm. in diameter, and jutting 
out from the stele to a distance of about 9 mm. In the next two sections there is little 
change; III. is the first figured (PI. 7. fig. 1); on the right-hand side of the projection 
a slight indentation will be observed, and above this, towards the extremity, a small 
portion at the corner is beginning to detach itself (see the enlarged fig. 7, aa, PL 8). 
In IV. this minor meristele is nearly free; it measures 9:6 x 1:5 mm. 
Between IV. and V. there is a gap, where longitudinal sections were cut; the interval 
between sections IV. and V. in the region under consideration was at least 1 em. " 
and it happens that a perceptible change of structure takes place. In V. the minor 
meristele has departed, the promontory showing a corresponding change of shape; in 
all probability the former has fused with the end of the long meristele to the right, 
which here shows an extension towards the left (Pl. 7. fig. 2). 
The main projection « appears hooked over towards the right, owing to the deepening 
of the incision on this side. A very definite group of small tracheides is seen embedded | 
in the interior of the projection about 4'5 mm. from the end; they are arranged in a | d 
ring or horseshoe enclosing a little nest of parenchyma, and the smallest elements are — 
only 32 u in diameter; from these there is a gradual transition to the full-sized 
tracheides of the surrounding wood, which are very large, ranging from 170 to 320 » in 
diameter. The position of this group at first appears to indicate an internal protoxylem, 
but everywhere else the structure is exarch, so this interpretation is not admissible. — 
The explanation suggested itself that this embedded group might be the downward | 
prolongation of a protoxylem-strand, which at a higher level lies at the periphery of the | 
meristele-xylem, on its inner face. The succeeding sections confirm this view. ; 
In VI. and VII. there is little change, but the incision on the right becomes deeper — 
and approaches the protoxylem-group. In Section VIII. (figs. 3 & 8) the meristele 
is getting near separation, the deep incision on the right corresponding to one 
on the opposite side. The embedded protoxylem (Pl. 8. fig. 8, pz.) has here become 
nearly peripheral on the inner surface. On the free surface of the meristele there is à 
marked depression, exaggerated, no doubt, by damage, but indicating a further division, 
as the next section shows. | 
Between VIII. and IX. there is an interval, probably representing some of the 
transverse sections which have not, passed through my hands. 
In IX. the meristele & has split into two unequal parts, and the larger, measuring 
6 5x5 mm., has now separated from the stele, while the smaller strand to the left 
(under 3 mm. in diameter) is not yet quite free (Pl. 7. fig. 4). 
* Judging from the form of the longitudinal secti 
sections IV. and V. are not accurately parallel. 
ons, the piece cut out was wedge-shaped, so that the transverse 
