McNicol. — On Cavity Parenchyma and Tyloses in Ferns . 407 
disintegrated protoxylem vessels. In longitudinal sections these cells are 
seen to be of various shapes, generally having a more or less spherical 
form, and frequently having an elongated vertical portion attached. In 
Fig. 9 two of these portions are shown cut across at et. The tissue is formed 
in the usual way by the intrusion of the cells of the xylem sheath into the 
spiral vessels, which are thus disintegrated. In only one instance did 
I find parenchyma cells intruding into a wide pitted tracheid, and in this 
case the intrusive swellings were only small, and the markings on the 
tracheid-wall were for some reason unlignified. The other tracheids 
adjoining the unlignified one were normal in character ; possibly the 
tracheid had become injured in some way, and so was weakened. The 
lignified cells of the cavity parenchyma are usually not actually in touch 
with the xylem of the vascular bundle, but may be isolated among a 
number of parenchyma cells. Fig. 10 shows a group of these thickened 
cells of the cavity parenchyma in longitudinal view : as a rule they do not 
occur quite so many together. The cavity parenchyma cells are not 
lignified until the tissue has developed to a considerable extent and 
the spiral vessels are broken up : they do not become lignified when they 
are protruding as bladder-like processes into the lumina of the vessels : 
such lignification would, of course, prevent the further expansion and growth 
of the cell. Miss Jordan (’ 03 ) found that in an herbaceous stem of Cucumis 
sativus there occurred tyloses having lignified walls, this lignification 
preventing the tyloses from extending far into the vessels, so that they 
never completely closed up the passages. The tyloses she figures resemble 
in appearance the lignified cells of the cavity parenchyma of Cibotium , 
which are formed in the first place by a process of tylosis, the difference 
being that whereas in Cucumis they are found actually within the lumen 
of the vessel, in Cibotium they have taken the place occupied by the spiral 
vessels of the protoxylem. The cells in Cibotium recall to a slight extent 
the water-storing cells or transfusion tissue found in certain fossil plants, for 
example, in Megaloxylon , and the centripetal wood occurring in the 
peduncles and petioles of recent Cycads. 
In the Hymenophyllaceae cavity parenchyma has been mentioned 
by Boodle in Trichomanes Prieurii , while absent in other species. In 
Hymenophyllum I found that though there was no trace of tylose-like 
formations, the spiral elements were broken up and the parenchyma cells 
enlarged ; the narrow woody elements seemed to be broken up by the 
pressure of the swollen parenchyma cells outside, not by the pressure 
of intrusions into the lumina of the vessels ; such a distinction in the 
formation of cavity parenchyma is, of course, one of degree only. In the 
Gleicheniaceae and in the Osmundaceae in Osmunda and Todea , cavity 
parenchyma has been recorded, as mentioned above. In the last named, 
the intercellular spaces in the tissue are large ; the protoxylem seems to 
