Tissues in Certain Monocotyledons. 29 
tubes become recognizable by their denser contents, and the 
perforation of the sieve-plates can be traced. It is important 
to recognize the phloem at the earliest stage possible, as 
otherwise the young sieve-tubes might easily be taken for 
tracheides developing by cell-fusion, a mistake which we 
suspect has actually been made. 
The careful study of serial tangential sections appears to us 
decisive against the hypothesis of cell-fusion, and in favour of 
that of sliding-growth. As regards the former point, the 
negative evidence is in itself very strong. More than half the 
bundle consists of tracheides. If they arose by cell-fusion 
therefore, this must take place on a great scale. In tangential 
sections through the middle of young bundles we should find 
most of the cells in various stages of fusion. Even supposing 
the fusion to take place so rapidly as to have been always 
missed by us (a very improbable supposition considering that 
the whole thickness of the growing zone was examined in 
many different series of sections), still at one stage we should 
find the xylem consisting of separate merismatic cells in 
rows, and at the next stage we should find it chiefly made up 
of continuous tubes, which must correspond exactly to the 
previous rows of cells. Either the nuclei must disappear or 
fuse, or they must be found in large numbers (thirty-seven on 
the average) in each tracheide. They certainly do not disap- 
pear till very late ; for a long time we find a single large 
nucleus in each tracheide, which is even more conspicuous 
than those of the other cells. Nor will fusion explain this. 
The fusion of more than thirty nuclei in a vessel more than 
thirty times the length of its constituent cells could not 
possibly be a quick process, and its stages certainly could not 
escape observation. Nothing of the kind was ever seen. 
The stages we actually find are: (1) the regular rows of 
desmogen-cells ; (2) the same arrangement, but slightly dis- 
turbed here and there by the presence of a longer element 
among the shorter ones ; (3) more disturbances of the primi- 
tive regularity, more elongated elements, the shorter cells 
more and more enveloped by them, until ultimately all the 
