364 Clark. — Secondary Thickening in 
In Fig. 8 the ingrowth has taken place mainly from the cell (d), and 
here the tracheid (e) is the only one of its row which has escaped destruc- 
tion. From comparison with other examples of similar cases, it is highly 
probable that tylosis into the missing tracheids first took place, and that 
these tylosed cells, partly by their own expansive force and possibly partly 
by absorption, caused the complete disappearance of the row of tracheids, 
of which (e) is the sole representative. No trace of the lost tracheids is to 
be seen, with the exception perhaps of an occasional slightly pronounced 
thickening of a wall of one of the cells of the new ingrowth. This, how- 
ever, may also be present, in cases where there is no suspicion of tylosis 
having occurred. 
In Fig. 9 the processes seem to be coexistent. The cell marked (a) in 
the figure has evidently been a tracheid into which tylosis has taken place, 
and subsequently its contents have overstepped their bounds and pene- 
trated between two adjoining rows of tracheids. In Fig. 10 rather a good 
example of this process is seen in a somewhat earlier stage. The two 
cells (a) are probably parenchymatous cells ; cells (b) and (c) seem to be 
tracheids, into which tylosis has taken place, and which have undergone 
a slight expansion. A small process from cell (c) indicates -the point at 
which penetration between the two neighbouring tracheids is about to take 
place. In Fig. 11, where the process has advanced somewhat farther, 
a solitary row of tracheids is represented, completely separated by paren- 
chymatous ingrowths from the rows adjoining. 
It is rather remarkable that many of these newly formed parenchyma 
cells develop farther into sclerotic cells, with peculiarly well-marked lines 
of stratification and pit-formation. As a general rule these sclerotic cells 
seem to be dotted about haphazard through the new growth ; sometimes 
surrounded by parenchyma-cells, as in Fig. 11, and sometimes inside 
tracheids or vessels, as in Fig. 13. These cells may possibly be of some 
use in the splitting process, by acting as blocks, holding apart the separated 
masses of xylem, and thus assist in relieving the pressure on the ingrowing 
parenchyma, which must of necessity be very great. This suggestion, 
however, will not hold good in every instance, as is obvious from the 
position of many of the cells. 
The ingrowth, as a general rule, starts from the two oldest, diametri- 
cally opposed wedges, and proceeds towards the pith (Fig. 15), but later the 
younger, less deep wedges, and even many of the isolated parenchyma 
patches follow suit. 
The result of the process is the division of the axial woody ring into 
a varying number of small portions, which lie embedded in a mass of large- 
celled parenchyma (Fig. 14). The arrangement of the separate portions is 
by no means regular, and varies in different parts of the stem. Occasionally 
very small fragments of wood, containing only a few tracheids, are found 
