454 
Saunders.—A Reversionary Character 
a central fibrous strand in the septum, a character regarded by systematists 
as of diagnostic value. In Mattkiola incana , R. Br., the main features to be 
noted in a transverse section of the developing siliqua are as follows (Fig. 11): 
1. The outer contour, which shows two larger lateral arcs formed by the 
two valves, and two intervening smaller arcs (commissures) bounding the 
ends of the single median partition. 
2. The central larger fibro-vascular bundle (midrib) of the valves, and 
the row of smaller bundles (lateral veins) on either side of it. 
3. The broad belt of lignified cells beneath the inner (lining) epidermis 
of the valves, which extends along the entire side of the loculus, and may 
even continue round the ends. A bridge of collenchyma eventually stretches 
across the thickness of the ovary wall from this thick-walled tissue to the 
outer epidermis at the point of junction of a large and small arc, thus 
delimiting the assimilating tissue of the two. When ripe the valves break 
away through rupture of the thin-walled tissue of the smaller arc where it 
abuts on the collenchyma and the lignified cells at the ends of the loculus. 
4. The distribution of the tissues in the smaller arcs and the connecting 
septum. The central region of the arc in the bud stage is occupied by 
a single median vascular bundle, or sometimes two equal bundles are found, 1 
while at or near the level of origin of an ovule an additional smaller bundle 
may be seen at the angle of the loculus. The number and arrangement of 
the bundles bears a relation no doubt to the ovule formation, and the not 
unusual occurrence in this species of two main parallel strands may be con¬ 
nected with the considerable width of the commissure, for by this arrange¬ 
ment the vascular bundle of the funicle has less distance to traverse before 
making contact than is the case when a single median bundle is present. 
In Cheiranthus Cheiri i L., where the suture is considerably narrower, 2 
a single central bundle appears to be the rule. As the fruit develops, the 
vascular tissue increases in amount; the several strands now become arranged 
more or less in a ring, the phloem of each bundle lying to the outside. 
A branch from the nearest strand connects with each funicle, and occasion¬ 
ally an anastomosis takes place round the angle of the loculus with a vein 
in the valve. We find in fact in the smaller arc a vascular supply quite 
equal to that of the valve, but compressed into a smaller area, the bundles 
lying on a small circle instead of an extended arc. Conversion of the neigh¬ 
bouring ground tissue into mechanical tissue eventually gives rise to a solid 
fibro-vascular cord. This cord is surrounded by thin-walled, loosely 
arranged cells, containing a little chlorophyll, which form the outer (basal) 
end of the septum. Following upon but sharply marked off from this looser 
1 The two conditions may occur at different levels in the same septum. 
2 The considerable width of the commissure in Alatthiola as compared with Cheiranthus is well 
seen in the drawings of the cross-section of the siliqua of these two genera in Reichenbach’s leones, 
ii, PL XLV. 
