240 Browne.—A Second Contribution to our Knowledge of the 
times gave off as many as seven superposed traces. In Cones A and B of 
E. limosum, a species in which the stele of the cone seems to be even more 
reduced than in E. maximum , I estimate the highest number of superposed 
traces given off successively by a single strand as respectively four and 
seven, the lower number being again characteristic of the cone with the 
better developed xylem. 
Medullary Tracheides. 
Stiles has already recorded the presence of groups of medullary tra¬ 
cheides in a branched cone of E. maximum (Stiles, pp. 114-16). I have met 
with medullary strands or small groups of medullary tracheides in three of 
the seven cones of E. maximum that I studied, i. e. in Cones A, B, and C. In 
the other cones I found no traces of medullary xylem ; but this by no 
means proved that these cones never developed medullary tracheides. 
The series of sections of cones other than Cones A, B, and E were not 
complete, and it is possible that some of these cones had medullary tra¬ 
cheides in parts of the cone. Moreover, except in young cones, or in the 
young parts of cones, the pith, or at least the central part of the pith, has 
perished, and small medullary strands may have perished too. Such strands 
would probably tend to persist somewhat longer than the soft parenchyma ; 
an illustration of this tendency is afforded by Cone B, in which the group 
of medullary tracheides is embedded in a projection of pith extending for 
a considerable distance into the central cavity. But where there is a very 
large central cavity, as in the lower parts of large mature cones of E. maxi¬ 
mum, medullary tracheides might well disappear, as do the tracheides 
developed in the position later occupied by the carinal canals, the destruc¬ 
tion of which often begins even near the apex, where the stele remains 
relatively narrow. Only an examination of a considerable number of cones 
in which the cells of the pith are still intact can settle the question of the 
relative frequency of the medullary strands in the cone ; but I am inclined 
to believe that in the species under consideration they are not uncommon. 
Speaking generally, the presence of medullary tracheides in a section 
is easily recognized, even by the naked eye, owing to the irregular agglo¬ 
meration of darkly coloured tannin-cells around them. A similar irregular 
concentration of large tannin-cells occurs on either side of the ring formed 
by the stele and all round the departing traces. Tannin-cells accompanying 
vascular tissue tend to have their long axes directed parallel to those of the 
tracheides. Besides these tannin-cells, associated with vascular tissue, 
isolated tannin-cells occur scattered more thinly throughout the pith and 
cortex. In all cases the outline of the medullary strands is shown in the 
longitudinal reconstructions by a broken line, black on the white and white 
on the black part of the diagram. 
In Cone A the medullary strand, though deeply seated, was slightly 
