330 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
cellular structures as those seen in fig. 27. If this has been the case then the possibility 
which I have suggested would become an established fact, viz., that so long as the 
young branch was passing outwards through the vascular zone of the parent axis, it did 
not acquire those features which we have seen to be so remarkably characteristic of 
Calamitean organization, but that when they did completely emerge from the stem 
those features appeared, at first in a very moderate degree, but soon undergoing 
rapid development both as to definiteness and size. It is impossible to believe that 
the large branch of fig. 30 was connected with its parent stem only by the tip of the 
small mamilla, a", constituting its inner extremity. There must have been a large 
amount of tissue of some kind uniting the two firmly together, but which tissues have 
disappeared during mi neralization — the only trace left of the former existence of that 
tissue being the dark stain, f which still connects the stem and branch, and which 
demonstrates that their union is organic and not accidental ; whether the stem and 
branch still continue at the same distance apart from each other that they held when 
living may be questioned — but all the numerous facts which we now possess show that 
a branch possessing a medullary fistular cavity of the dimensions seen in the specimen 
under consideration, must have possessed a vascular cylinder of considerable thickness 
to sustain its weight, even when we have made all necessary allowance for specific 
variations in the relative dimensions of these two parts in such plants. 
Since writing the above descriptions, I received from Mr. Isaac Earnshaw a speci- 
men which, on being cut into, revealed important evidence bearing upon the question of 
the branching of Calamites. A transverse section of part of the specimen is represented, 
two-thirds of the natural size, in fig. 31b, and a longitudinal section is shown in 
fig. 31a. At 31b, a is the medullary cavity of the parent stem, whilst at a of the 
same figure we have the similar cavity of a lateral branch, m. The vertical section, 
fig. 31a, shows that the branch, a ', is given off from the stem, a , nearly at right angles 
to the latter ; we here learn many truths : 1st, that the medulla of the smaller branch 
is fully as large as that of the parent stem, as is the case in the specimen fig. 30 ; 
2nd, that the woody wedges, with their large primary medullary rays, are equally 
developed in both ; 3rd, that only the outer and newer exogenous vascular layers of the 
parent stem extend over and form the vascular zone of the branch ; 4th, at i, i, 
Ave have the two sides of a node of the parent stem — being that from which the 
branch obviously originated — and at i , i', we have the next superior node of the same 
axis. On the branch I discover no trace of a similar node. These two sections clearly 
prove, beyond possibility of further doubt, that whatever may have been the condition 
of the lateral branches in their very young state, such branches as did not fall off or 
become abortive, gradually became invested by the successive woody layers that were 
added to the parent axis subsequently to the first appearance of such branches. 
The two grew together, and thus the relations of stem and branch became exactly the 
same as those observable in any ordinary exogenous tree. It thus becomes evident 
that the correctness of my argument in favour of my opinion, viz., that the medullary 
