690 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
for it the separate specific name of Arpexylon simplex. I have now, however, indis- 
putable evidence that the two are parts of the same plant, and that fig. 39 is but a 
secondary petiolar branch of fig. 28. I have traced these branches downwards, exhibiting 
every degree of size from fig. 39 to fig. 40, and even further, since I find abundance of 
yet smaller twigs in some of my slides, which I have no doubt are the terminal portions 
of the same structures. The latter have but a very few vessels forming a small central 
bundle, which loses its special crescentic shape. These conditions remind us of those 
already represented in figs. 22, 23, & 24, belonging to Rachiopteris Oldhamia. 
The last described sections make it quite clear that the two halves of the subdivided 
crescentic bundle seen in Plate LY. fig. 35, a!, a', have been so subdivided preparatory to 
passing outwards through the bark to supply secondary branches like fig. 39. A very 
slight modification of each of these two clusters of vessels in the former figure would 
give to it the characteristic form of fig. 39, and the entire process would be in close 
accordance with what takes place amongst recent ferns. 
The difficulties attending the interpretation of this petiole are very great, and would 
have been much more so, but for a series of sections for the loan of which I am 
indebted to my friend Mr. Carrutheks. The obvious question suggested by the speci- 
mens which I have already described is this — have the secondary rachides been invariably 
given off from one side of the central hour-glass bundle 1 or has the primary crescentic 
bundle [a!) been formed and disappeared in the shape of pairs of secondary bundles, 
alternately given off from its opposite sides \ The specimens now to he described 
demonstrate the truth of the last of these suggestions. 
Plate LY. fig. 35 A is part of an imperfect transverse section of a matured petiole. 
At a we have the two extremities of the hour-glass bundle, whilst at aa, aa we have, 
passing through the cortical layer, two crescentic secondary bundles, each of which 
have been exactly like that of fig. 39 ; on the concave side of the perfect crescentic 
bundle {aa), on the right of the figure, we have a yet smaller one (x), which has 
evidently been detached from that with which it is so closely associated. In fig. 35 B, 
which represents a second section of the same petiole, taken a little higher up, we 
obtain a clue to the relations of these several parts. At a we have the hour-glass 
bundle accidentally ruptured across its central portion ; at a! we find the large primary 
crescentic bundle in its normal position as a single unbroken band. Longitudinal 
sections in my cabinet clearly demonstrate that the vessels of this primary crescentic 
bundle leave those of the hour-glass one very gradually, since the two sets run for 
considerable distances in perfect parallelism with each other. At aa, aa we find the 
two secondary crescentic bundles going off to some secondary lateral appendages, clearly 
demonstrating the fact that those appendages, whatever they have been, were alternately 
given off from the opposite convexities of the hour-glass bundle. The groove at a w is 
nearly obliterated, in consequence of the recent detachment of the primary crescentic 
bundle a' ; but the opposite side has so far recovered from the effects of the detachment 
of the secondary bundles aa, aa, that, instead of being truncated, it has regained its 
