DR. D. H. SCOTT ON BOTRYCHIOXYLON PARADOXUM. 375 
while the surrounding xylem-zone is of great thickness, with as many as 20 elements in 
aradial row. In the section above the dichotomy (Pl. 38. fig. 7), the best for details, the 
“mixed pith” is nearly isodiametric (0:4—0:5 mm. in diameter) and the radially arranged 
wood less extensive, reaching about 10 elements in thickness. It is probable that the 
flattened, irregular shape of the central tissue in the lower sections is in part due to 
pressure set up by continued secondary growth, as happens in the case of the pith of 
Aristolochia, among living plants (De Bary, '84, p. 533). The inner margin of the zone 
of wood shows five or six slightly convex protrusions towards the mixed pith, which 
extends outwards to a varying extent between them, so that its outline is somewhat 
fluted or crenulated (see Pl. 37. fig. 1, Pl. 38. fig. 7, and Pl. 41, fig. 20). At certain 
points the angles of the central tissue are prolonged outwards for a considerable distance 
~ into the zone of wood, sometimes extending through its whole thickness, so as to form 
apparent rays. It appears, however, that these ray-like prolongations contain tracheides 
aud are connected with the emission of strands to the various appendages of the stem. 
It is doubtful whether true medullary rays are present; if so, they must be very few in 
number. Transverse sections afford no decisive proof of their existence; in the radial 
section (Pl. 39. fig. 10) there appears to be a true ray, with square cells, at m.r., but one 
cannot be certain that the structure may not have been connected with some outgoing 
strand. Tangential sections are needed to settle the question. 
The radial seriation of the tracheides is quite regular, so that the tissue presents all 
the appearance of a zone of secondary wood. In the best transverse section the cambial 
cells and young xylem-elements are very clear, especially at one point (Pl. 38. fig. 7, cb.). 
There thus seems to be no doubt that the radially seriated zone really represents a produet 
.. Cf secondary growth carried on by means of a cambium-layer. This is confirmed by the 
diminution in thickness of the zone of wood in the upper part of the stem, while the 
primary central tissue maintains an almost constant size throughout. Little, if any, 
secondary phloem appears to have been added by the cambium. : 
The uppermost section showing the stele has a peculiar structure, no doubt dus y 
pathological (Pl. 39. fig. 8). There is here a considerable formation of wound periderm 
in the cortex, and on this side the stele is incomplete, for there is no secondary wood, 
though on the other side it is well developed, reaching a thickness of about 8 pet 
It may be pointed out that the effect of the injury has apparently €— p" ? e 
formation of the secondary tissues; their presence is evidently —— E. coli 
In all the sections the nature of the “ mixed path” is clear; ^ mea Pa 
tracheides intermingled with parenchyma. The structure is best ee a ie 
transverse sections (Pl. 38. fig. 7, Pl. 39. fig. 8, Pl. 41. fig. 20). The z € s 
range, as a rule, from about 30-50 u in diameter, but the gee 4 ie ind 
only 12-24 u; the secondary tracheides range about 40-80» n Sere p the mixed pith 
predominating. Of the internal tracheides, those towards the rd pim places groups 
are usually the larger. Tt is not easy to locate the dein 3 the outside of the 
of very small tracheides which are probably of this nature He on ;. 90 
i SP dary zone (see Pl. 39. fig. 8, Pl. 41. fig. : 4 
primary wood near the beginning of the secondary h ne such group (pz.) is 
This is well seen in the uppermost section (Pl. 39. fig. 8) mere 2 Sx : 
