353 MR. M. D. ZALESSKY ON NEW 
oceur in the pericyele of living Osmundacex. At this level and sometimes 
at a lower one may be seen the splitting of one protoxylem group into two. 
At a higher level the vascular bundle becomes more and more open on the 
adaxial side; at first it lias a narrow falcate outline, and later assumes the 
form of a horseshoe (Pl. 82. fig. 5). The number of protoxylem groups is 
increased by the branching of the two original groups until in a leaf-trace 
on the point of passing from the inner cortex to the outer the number of 
protoxylem strands may be four or five. Asa leaf-trace opens on the adaxial 
side the bay-like prominence of tissue surrounding it widens, and in this 
prominence it is possible to detect a band of phloem accompanied on the 
outside by a series of large cells with black contents (Pl. 32. fig. 6). Asso- 
ciated with the small-celled tissue of the pericycle are some parenchymatous 
cells of the inner cortex. The latter tissue may be accompanied by a thick- 
walled tissue having the features of the outer cortex. The large cells with 
black contents which make their appearance in the leaf-trace as it passes 
through the inner cortex occur as a group on its adaxial side. In the peri- 
cycle they appear to be characteristic of that tissue as it traverses the outer 
cortex, and especially when it passes through the petiole. In transverse 
section these cells appear to be vesicular, and in longitudinal section they 
resemble much elongated, septate fibres. Kidston and Gwynne- Vaughan 
call these cells mucilage sacs, and compare them with similar elements in the 
pericycle of living Osmundaceæ. They are undoubtedly mucilage sacs, and 
agree closely with those in the pericycle of recent Osmundacex. They are 
situated on both the adaxial and abaxial sides of the leaf-trace, along which 
the phlocm forms an investment to the horseshoe-shaped bundle of the xylem 
immediately behind the protophloem. In the petiole the pericycle, which is 
feebly developed, as we have scen, in the leaf-trace during its passage from 
the inner cortex, becomes more developed and consists of five layers of small 
cells. The mucilage sacs are associated with these cells. On the abaxial 
side of the leaf-trace they are usually arranged in one or two series ; on the 
adaxial side, where the pericycle zone attains a greater thickness, they are 
more irregularly arranged. On both the abaxial and adaxial side of the leaf- 
trace these arcs and mucilage sacs are enclosed by resistant cells which are 
stained brown, but agree closely with the black resistant cells which form the 
sheath of the leaf-trace in the inner cortex. As the fundamental tissue of 
the petiole is parenchymatous and light in colour, it is easily distinguished 
from the brown resistant cells surrounding the leaf-trace, and from the cells 
of the sclerenchyma which extend to the periphery and form in the petiole a 
border of characteristic outline. Outside this sclerenchymatous border is 
a narrow band of parenchyma which marks the limit of the petiole. This 
band represents the wings of the leaf stipules: the boundary between con- 
tiguous wings is indicated by a brown line. The wings of the stipules of 
