1883.] Anomalous Oolitic and Palceozoic forms of Vegetation. 227 



nous growth. But these regular conditions have frequently been 

 disturbed; first by the cambium ring extending itself partially or 

 wholly around one or both of the free paired bundles, e, leading 

 either to a one-sided development of exogenous vascular laminae on 

 their external sides, as at Fig. 7, e', or sometimes, as in/, entirely sur- 

 rounding a bundle with a complete cylinder of radiating laminae. 

 But further irregular developments have sometimes taken place. 

 Some of the cellular medullary rays separating the vascular laminae 

 of the zone c have undergone great enlargement, breaking up the 

 j zone into several segments of a circle ; and in one such example in 

 my cabinet, the cambium has extended itself centripetally round the two 

 converging sides of each vascular segment, and formed a boundary line 

 between its inner angle h and the pith a. The consequence of this has 

 been the unwonted development of new vascular laminae, which pro- 

 ject centripetally into the pith from the cluster of non-radial vessels h. 



This tendency to a multiplication of independent centres of 

 exogenous growth within the area of an enclosing bark, inevitably 

 reminds the botanist of the not wholly dissimilar conditions charac- 

 teristic of some Sapindaceae, such as Paullinia and Sejania. Only 

 what is a variable feature in the Lyginodendron, is in these latter 

 plants a regular and normal one ; but the strong tendency to such 

 peculiar variations in an archaic type may easily be conceived to have 

 led to the ultimate production of more constant differentiated forms. 



A second remarkable and anomalous stem from Burntisland, in 

 Fifeshire, is described in the same memoir as the Lyginodendron, 

 under the name of Heterangium Grievii. This plant must have been 

 sufficiently abundant to have formed a conspicuous object in the Car- 

 boniferous forests of that portion of the primaeval world ; yet we are 

 as ignorant about its foliage, fructification, and botanical relationships 

 as we are of those of the Lyginodendron. 



In the ' Philosophical Transactions ' for 1878 I described, under 

 the generic name of Asteromyelon, the central portions of the stem and 

 branches of a plant that was very common in the carboniferous area 

 of west Yorkshire, eastern Lancashire, and the intermediate Pennine 

 Hills. The discovery of its curious bark two years ago was made the 

 occasion by Messrs. Cash and Hicks of giving to this plant the 

 specific name of Williamsonia. In the general plan of its organisa- 

 tion this plant has many features in common with that of the living 

 Marsileae. This is especially the case with the bark ; but it exhibits 

 extraordinary variability in the structure of its central axis. Some- 

 times the actual centre is occupied by a vascular bundle from which 

 cellular elements are almost, if not wholly, excluded. In other cases 

 this centre is occupied by a very large and beautiful pith, transverse 

 sections of which present the exquisite stellate form that led me to 

 give to the object its generic name of Asteromyelon. We have here 

 another example of an organism exhibiting great variations of struc- 

 ture, due partly, no doubt, to differences of function, but partly to 

 difference of surroundings. 



