352 G wynne- Vaughan. — On a ‘ Mixed Pith ’ in an 
medullary tracheae distributed quite irregularly over the whole area of the 
pith. They occur as isolated elements or in small groups up to as many as 
nine together. In a few cases they are seen in contact with the tracheae 
of the normal xylem ring, but they do not extend into the medullary rays. 
As seen in transverse section they are very irregular in form and vary much 
in size, closely resembling in these respects the parenchymatous cells of the 
rest of the pith. A few, however, are somewhat larger and more or less 
vesicular or sack-like in appearance. Since the surfaces of the end walls of 
most of them are visible in a transverse section they must be quite short in 
the vertical direction. Indeed, some of them seem to be broader trans- 
versally than they are long (Fig. i). The elements in question are entirely 
tracheal in character. They are quite dead and empty, with thick and well- 
lignified walls. Their pitting is irregularly scalariform, passing over in many 
cases to a reticulate or even a porose marking (Fig. 3). A very similar 
reticulate-porose marking is also characteristic of the medullary tracheae of 
Osmundites Kolbei (Fig. 4). 
The whole length of the stem, which was about 55 mm. long, was care- 
fully cut into successive transverse sections, but no more medullary tracheae 
were encountered until a point was reached about 8 mm. from the apex, 
where they appeared once more and persisted for about a dozen sections. 
Above this point the stem was cut longitudinally, but did not disclose any 
medullary tracheae. In this upper anomalous region the medullary tracheae 
are essentially the same in character as those in the lower region, but they 
are fewer in number, not exceeding nine in any one transverse section 
(Fig- 5)- 
The presence of this unusual type of pith in our specimen is un- 
doubtedly due to injury. Indeed, the pith is obviously in an unhealthy 
condition throughout the whole length of the stem. It is traversed in all 
directions by cracks which in alcohol material are filled with a yellow 
mucilaginous substance. They are formed partly by the tearing apart of 
the pith cells and partly by the breaking down of the same, and are usually 
bordered by the remains of the disintegrated cells (Figs. 1 and 5). It is 
seen that the medullary tracheae are mostly found in close relation to these 
cracks, from which in Fig. 5 the mucilage has been removed by the action 
of eau de Javelle. 
It is not clear whether the plant was suffering from some constitutional 
disease or whether it had received a definite external wound. The fact that 
there is no trace of such a wound and that the anomaly reappears in a region 
just below the apex seems to point to the former suggestion. At the same 
time a traumatism may appear at a considerable distance away from the 
wound that causes it, and in this case the wound may have been inflicted on 
a part of the stem below that collected. The lower anomalous region may 
have been formed shortly after the reception of the injury, and since the 
