544 Brebner . — On the Anatomy of 
Fig. 25 illustrates a similar vein in longitudinal section, and 
proves that the protoxylem is really endarch. 
Finally, the occurrence of ‘ cavity parenchyma ’ in the leaf 
may be mentioned. A not particularly good example is 
shown at cp ., Fig. 22. The cavity due to the breaking down 
of the protoxylem is filled with parenchyma, owing to the 
increase in size, accompanied by a greater or less amount of 
division, of the adjacent living cells. It is practically a case 
of thylosis, only that the process is not accompanied by so 
much cell-division, and it is not the protoplasts but the 
cells as a whole that grow into the space. In this instance 
the internal xylem has been separated from the protoxylem 
in the process. It is probably not of any physiological 
importance, being simply a case of non-pathological hernia, 
so to speak. 
The Stem. 
In view of the recent work on the stem of Angiopteris and 
Marattia there is very little more to be said about its histo- 
logy. The meristeles, as has already been pointed out by 
Kuhn and others, whether of the dictyostele or the outgoing 
leaf-traces, are practically identical in structure in all the 
genera. Although not easily expressed in words, these 
meristeles have a sufficiently characteristic appearance to 
enable them to be distinguished from those of most other 
Pteridophyta. This is partly due to the absence of the 
endodermis round the outgoing leaf-trace meristeles of the 
stem, and its apparent 1 absence round the dictyomeristeles 
themselves. Besides finding it round the meristeles of the 
seedling, Farmer and Hill were able to demonstrate its 
presence in the older stem in Angiopteris , Marattia and 
Kaulfussia 2 . The fact that it is absent round the outgoing 
leaf-traces, after they have become quite free from the 
dictyostele, may account for the older observers having 
overlooked it, because they probably did not distinguish 
1 The endodermis, even where most marked, as in the seedling-stem, cannot be 
« detected without staining or other micro-chemical means. 
2 Loc. cit., p. 386. 
