5°8 
Boodle. — On the Occurrence of 
tracheides, to be regarded as secondary, scattered in the zone of parenchyma 
referred to above 1 , and hence outside the mass of tracheides forming the 
primary xylem. These additional tracheides are mostly separated by at 
least one layer of parenchyma from the central group, but some of them 
may be in contact with it. They are only to be found in old parts of 
the plant, where they may show all stages of differentiation, but in the 
oldest parts examined 2 all the secondary tracheides present were com- 
pletely lignified. 
Figs. 2-6 in PI. XXXIII show secondary tracheides in the tissue 
surrounding the primary xylem. Text-Fig. 44 is an external view of the 
stem from which these sections were cut. In this figure, e y f, and g are 
three aerial shoots, and it is clear from the disposition of the parts that 
e must have been the first-formed aerial shoot, and that from its lower 
region an obliquely ascending branch ( c , d ) grew out on the right and gave 
rise to the two other aerial shoots f and g. 
The whole of the branch c y d should perhaps be classed as the stock of 
the aerial shoots, the lower part certainly so, but, on the right beyond the 
point of insertion of f it becomes rhizome-like again, as evidenced by 
reduced diameter, remains of rather numerous rhizoids, and by the character 
of the stele. A transverse section from this region, cut at the level of 
d in Text-Fig. 44, is represented in Fig. 2, PI. XXXIII. It will be seen 
that there is a centrally placed group of tracheides (p), probably diarch, 
which is comparable with the primary group in Fig. 1, PI. XXXIII, though 
the size and number of the elements differ. In Fig. 2 the central group of 
primary tracheides (p) is surrounded by a considerable number of smaller 
tracheides, which are to be regarded as of secondary origin. Several of these 
elements have much darker walls than the tracheides of the primary group. 
This is due to the colour-differentiation of the stain employed (methyl-green 
and eosine), all incompletely lignified walls having taken up eosine as well as 
methyl-green, the combination of the two stains producing a purple colour, 
which is in strong contrast to the green or blue of the fully lignified 
elements. Watery methyl-green followed by alcoholic eosine proved 
a very satisfactory stain for differentiating such partially lignified elements 3 . 
Phloroglucin with hydrochloric acid was used for comparison; two con- 
secutive sections were chosen, and one was stained with methyl-green and 
eosine, the other with phloroglucin. A close correspondence was shown ; 
the elements which stained purple or purplish black with the double stain 
being just those which only stained faintly with phloroglucin, while all the 
1 Or largely replacing it. 
2 Probably one year or more old. Proportional age may be estimated by the amount of the 
well-known brown substance, which encroaches on and gradually fills up the cavities of many of the 
cells of the inner cortex, and by the hardness of the specimen. An old branch of the rhizome showed 
seven primary and twenty secondary tracheides, all mature. 
3 This stain was used for the same purpose in Ophioglossum (Boodle, ’99, p. 386 ). 
