Anatomy of the Ophioglosseae. 387 
which the drawing was made, this tracheide was well differen- 
tiated from the others by the staining of its walls. 
These developing tracheides must, it seems, be regarded 
as secondary, as the addition of such elements takes place in 
roots several years old. Thus the part of the root shown in 
Fig. 9 was cut close to the attachment of an adventitious 
plant, on the side towards the parent plant ; and, judging 
from the number of young leaves on the adventitious plant, 
this part of the root must have been four or five years old. 
The secondary tracheides are only formed in small numbers ; 
and, as they are added one here and one there, there is no 
definite cambial layer, though a newly divided cell may 
occasionally be seen with its thin division-wall, as in a cell 
bordering on the xylem in Fig. 10. 
The greatest addition of secondary elements was seen in 
the region of the root near the base of an adventitious plant. 
Now and then the outermost tracheides show radial arrange- 
ment ; in many cases no doubt these elements are secondary, 
but their arrangement does not give a safe criterion, as 
occasionally the xylem may have a roughly radial disposition 
from the first. 
The root-material examined was collected in June and 
December. As one would expect, no good cases of develop- 
ing tracheides were found in the December material. 
The stem also was examined for secondary thickening, 
with the result that it showed an exactly similar addition of 
tracheides, though this was more difficult to observe on 
account of the oblique course of the bundles. The tracheides 
here appeared to be added on the outer side of the wood 
only, while in the root they may be added on all sides. Fig. 
13 shows part of a ' stem-bundle in which there are five 
developing tracheides on the outer side of the xylem. This 
bundle must have been differentiated four or five years ; and 
developing tracheides were seen in one or two still older 
bundles. 
Sections of the young stem show that the leaf-trace bundles 
arise as distinct procambial strands, and that the first-formed 
