Comparative Anatomy of the Casuarineae . 247 
where they abut on one another or on vessels or tracheides 
there are always bordered pits on the common wall ; where 
they abut on parenchyma-cells, however, there is always 
a half-bordered pit on the common wall. Strasburger 1 and 
Sanio 2 , in writing of other cases investigated by them, cite 
the two facts of the occurrence of bordered pits on the wall 
separating two ‘fibrous tracheides/ and the formation of 
parenchyma among these elements as evidence of their 
tracheidal nature. The occasional occurrence of a tertiary 
spiral in these elements points to the same conclusion. 
Ordinary tracheides likewise occur, though they are few in 
comparison with the other elements of the wood: Fig. 28 
shows one near the pith ; on its outside is a row of paren- 
chymatous elements, which in their turn abut on a fibrous 
tracheide. Transitional forms between the tracheides and 
the fibrous tracheides occur. 
In one unnamed species a very interesting form occurred, 
which exhibited very clearly the mode in which the slit-like 
perforations of the vessels have their origin : at each end of 
the terminal wall of the tracheide bordered pits were irregu- 
larly scattered ; but towards the centre of the wall they 
gradually grouped themselves in transverse rows of two or 
three, at first unconnected with each other, afterwards par- 
tially united, till, finally, what appeared as one large slit-like 
bordered pit was formed by the complete fusion of two or 
three small ones (Fig. 29). 
In some sections made on that side of the stem, where 
a branch had lately passed in, a longitudinal series of trach- 
eides was observed, which were running irregularly down in 
the broad zone of parenchymatous tissue described above 3 . 
They possessed dense spirals and pointed ends, and were 
evidently protoxylem elements. These are the same ele- 
ments which have been described above as seen in trans- 
verse section. As we have also seen, they were mentioned 
1 Ueber den Bau und die Verricht. der Leitungsb. in den Pflanzen, 1891. 
2 Loc. cit., 1863, pp. 115, 1 16. 
3 Above, p. 243. 
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