828 HICK AXD CASH : AFFINITIES OF LEPIDODENDRON. 
than one vascular bundle oft'er no objection to this. In them the 
bundles are isolated, whereas in LycoporVi am they are fused, but there 
seems no reason for doubting that the sheaths of fused bundles delimit 
the same groups of tissue as the sheaths of the bundles themselves. 
Assuming that these general homologies will not be contested, 
the comparison of the stems under consideration may be carried into 
greater detail. AVhen this is done, it is found that there are certain 
agreements and difterences between them that may be formulated as 
follows : — 
A. Lepidodendron axd Lycopopium. 
1. Agreements. 
i. Both are covered by numerous, relatively small leaves, 
which are inserted on all sides, 
ii. Both have an axile vascular cylinder, which has probably 
arisen by the fusion of a number of vascular bundles. 
iii. In both, the transverse section of this cylinder is circular, 
a feature wliicli is doubtless to be correlated with the 
phyllotaxis, the leaf-trace bundles coalescing with the 
cylinder on all sides. 
iv. In Lycopodium, the development of the xylem plates is 
centripetal. This holds also for Lepidodendron Spencerl, 
Will., as described by Williamson in his tenth memoir. 
The appearance of the primary xylem of other species of 
Lepidodendron suggests the same order of development, 
but positive proof of this is still wanting. In both 
genera, however, the smallest elements are at the peri- 
phery of the cylinder, and the larger ones internal. 
V. In young shoots of both Lepidodendron and Lycopodium 
the cortex appears homogeneous in composition, the 
hypoderma being merely distinguishable topogTaphically 
from the inner parenchyma, 
vi. In both again, the hypoderma passes gradually into the 
tissues of the leaf. 
2. Differences. 
i, Lycopodivm has no pith enclosed within the axile 
vascular cylinder. The same may be said of fxpido- 
