XVIIl] GENERAL CONSIDERATIONS 269 



Lady Isabel Browne', who has recently published an excellent 

 summary of the evidence on the relation of the Lepidodendreae 

 to Isoetes, concludes her examination of the arguments by 

 expressing the opinion that there is a strong probability of the 

 correctness of the view that Isoetes may be derived " from the 

 Lepidodendraceae in the widest sense of the word." This 

 decision seems to me to accord best with the facts. 



The further question as to the relation of these Palaeozoic 

 genera to plants higher in the scale must be reserved for 

 fuller consideration in another volume. An attempt will also 

 be made to consider how far anatomical structure may be used 

 as a guide to the conditions imder which Lepidodendron and 

 Sigillaria as well as other members of the Permo-Carboniferous 

 floras passed their lives. The secondary xylem of Lepidodendron 

 and Sigillaria affords a striking example of water-conducting 

 tissue of homogeneous structure comparable with the wood of 

 Conifers rather than with that of Angiosperms. It was pre- 

 sumably formed, for the most part, under uniform climatic condi- 

 tions : the absence of rings of growth points to uninterrupted 

 supply to evergreen shoots exposed to no alternation of activity 

 and arrested growth. Attention has already been called to the 

 absence of any tissue corresponding to secondary phloem . Even in 

 young shoots of Lepidodendron, no tissue has been found external 

 to the meristematic zone agreeing in the form of its elements with 

 the channels through which the elaborated food is conveyed from 

 the leaves of recent plants to the regions of cell-building. That 

 the ' secretory zone ' may have served this purpose, at least in 

 young stems, is not improbable. On the other hand, it is 

 difficult to understand why older Lepidodendron stems show 

 no indication of additions to the secretory zone. If this tissue 

 served for the transport of proteids we should expect to find 

 provision made for its constant renewal pari passu with the 

 secondary growth of the xylem. The conclusion seems to me 

 inevitable that the supply of building-material was otherwise 

 provided for than in recent vascular plants. The physiological 

 division of labour may have been less complete in the tissue- 

 systems of the Palaeozoic Lycopods than in the more highly 



1 Browne (09) p. 37. 



