28 Scott and Brebner . — On the Secondary 
appear, and the cell-contents gradually vanish. As long, 
however, as the living body of the element persists, it possesses 
the single large nucleus by which it was at first distinguished. 
While the young tracheides are going through these 
changes the other elements of the bundle undergo at first 
little modification. In fact, the strand of desmogen remains, 
but becomes gradually enveloped (except at the extreme 
outer edge) and partly permeated by the hypha-like 
tracheides. The latter grow in diameter as well as in length, 
and so come eventually to form much the greater part of the 
mass of the bundle,— -a part altogether out of proportion to 
their number. 
The developing tracheides have at first a denser protoplasm 
than the neighbouring cells, a fact which often helps greatly 
in recognizing them. In the later stages the denser proto- 
plasm is found to be limited to their pointed ends, which we 
must suppose to be the seat of active growth. 
In order to observe the stages of development satisfactorily 
it is essential to have very thin sections. This has the 
disadvantage that the complete tracheide is hardly ever con- 
tained in a single section. Hence the necessity for serial 
sections from which the form of the whole element can be 
reconstructed. It remains difficult to find suitable prepara- 
tions for drawing. Fig. i is fairly satisfactory, but even here 
there is reason to believe that the extreme upper end of the 
tracheide is incomplete. 
The developing tracheides often have a somewhat crooked 
course, as seen in tangential section. This would in itself 
agree well with their origin by cell-fusion, as the desmogen- 
cells of successive tiers do not lie in the same straight line, 
but roughly alternate with each other. It is evident, however, 
that the same course must also result from sliding-growth, for 
the growing tracheides must undergo a bend whenever they 
pass the points of junction of neighbouring cells, which are 
not in the same straight line. After the development of the 
tracheides has made some progress, the phloem becomes 
differentiated at the outer side of each bundle. The sieve- 
