484 Watson . — On the Structure and 
diameter of a lepidodendroid stem may be expected as a result of secondary 
growth. 
It must be pointed out at once that the case is quite incomparable 
with any that we have an opportunity of examining in living material, as, 
except for the Lycopods themselves, no living plant has an anatomy of the 
general type of Lepidodendron , and all the angiospermous trees differ so 
absolutely in their structure as to give us no help. 
It seems certain that when once the wood and outer cortex of a lepido- 
dendroid stem were laid down and matured, their diameter was fixed, as 
additional tracheides or cells cannot have been introduced. 
It must be noticed that this fact, the fixity in dimensions of the primary 
outer cortex and wood when once matured, implies an enormous mass of 
living and dividing tissue at the growing point. y 
I have seen purely primary stems of the order of 15 cm. in diameter, 
and much larger examples must occur. The idea of a growing point of the 
order of 20 cm. in diameter is a somewhat remarkable one, but seems to be 
supported by impressions. Several specimens are known in which a branch 
whose diameter is of this order ends suddenly, the leaves forming a bunch 
all over it. A similar occurrence is known in Stigmaria. These cases seem 
to me to be real growing points, and not to be explicable by any injury. 
If we can regard the diameter of the outer cortex as fixed — and, so far 
as I can see, we must do so — it provides us with a datum by which we can 
measure the actual increase in diameter of the stem owing to its secondary 
growth. That we are justified in making this assumption is, I think, 
rendered very probable by the fact that, whilst, in stems with an appreciable 
amount of secondary cortex, the cylinder of primary outer cortex which lies 
outside it and carries the leaf-bases is usually split longitudinally, having 
been unable to stretch sufficiently to accommodate the growing trunk, the 
precisely similar ring of primary outer cortex within the periderm is never 
fissured or in any way injured. This view implies that the secondary 
growth of the wood has no influence on the diameter of the stem, being 
allowed for by the crushing up of the middle cortex (the volume of the 
secondary wood in most cases is so small in comparison with that of the 
middle cortex as to allow of this taking place). 
Accepting the diameter of the primary outer cortex as constant, it is 
easily seen that the increase in diameter of any lepidodendroid stem is twice 
the thickness of the secondary cortex. The thickness of the secondary 
cortex in proportion to the diameter of the stem is always small ; in large 
trunks, in which it is usually greater than in small branches, it may be as 
much as 9 cm. in a stem of 30 cm. ultimate diameter, as in the Dalmeny tree 
of L. Wiinschianum. This implies an increase of only one-half of the 
original diameter, an amount which seems to be about the maximum in the 
very large series of sections I have examined. 
