10 SPHENOPHYLLALES [CH. 



the central cylinder is not therefore absolutely constant, but 

 may be replaced by a mesarch arrangement. 



The presence of a few sterile leaves on the peduncle below 

 the fertile portion of the cone, which agree in their lobed 

 laminae with the sporophylls, is the only fact which we possess 

 as to the form of the vegetative characters of the genus. 



The above description is sufificient to indicate the extra- 

 ordinary complexity and high degree of specialisation of 

 Cheirostrobus. The sporophylls, with their trilobed segments, 

 and the crowded sporangia of exceptional length attached only 

 by a narrow base constitute striking peculiarities of the genus. 



It is unfortunate that we are still without any satisfactory 

 evidence as to the nature of the plant the cones of which have 

 been made the type of a new genus and a new family. 

 Cheirostrobus affords an interesting example of a type of 

 reproductive shoot constructed on a plan sui generis, and may 

 be classed with some other extinct genera as instances of the 

 production in the course of evolution of architectural schemes 

 which appear to have been ill adapted for competition with 

 equally efficient though much simpler types. But the discovery 

 of these isolated forms of restricted geological range among 

 the relics of the Palaeozoic vegetation frequently supplies a 

 key to phylogenetic problems. Cheirostrobios by its complex 

 combination of features characteristic of the Equisetales, the 

 Lycopodiales and the genus Sphenophyllum throws a welcome 

 light on the inter-relationships of groups which represent 

 divergent series. The combination of morphological features 

 in this generalised type led the author of the genus to describe 

 it as a descendant of an old stock which existed prior to the 

 divergence of the Equisetales and Lycopodiales. 



The discovery of this new type of strobilus naturally led to 

 a search among Lower Carboniferous plants for vegetative shoots 

 exhibiting characters conformable with the whorled and branched 

 leaves of Cheirostrobus. In Sphenophyllum we have a genus 

 obviously Comparable with Cheirostrobus as regards the form 

 and disposition of the leaves, but the differences between the 

 cones and the striking similarity of the vascular cylinder of the 

 latter to that of Lepidodendron demonstrate conclusively that 



