392 SPHENOPHYLLUM. [CH. 



which sometimes reach a considerable length. One of the 

 longest examples known is in the collection of the Austrian 

 Geological Survey; the axis is 4mm. in breadth and 85cm. 

 long, bearing a slender branch 61 cm. in length. The 

 manner of occurrence of the specimen as a curved slender 

 stem on the surface of the rock suggests a weak plant, which 

 must have depended for support on some external aid, either 

 water or another plant. The anatomical structure and other 

 features do not favour the suggestion of some writers that 

 Sphenophyllum was a water-plant S but there would seem to 

 be no serious obstacle in the way of regarding it as possibly a 

 slender plant which flung itself on the branches and stems of 

 stronger forest trees for support. 



I. The anatomy of Sphenophyllum. 



The following account of the structural features of the stem 

 and root is based on the work of Renault^ Williamson^ and 

 Williamson and Scott*. We may first consider such characters 

 as have been recognised in different examples of the genus, and 

 then notice briefly the distinguishing peculiarities of two well- 

 marked specific types. 



a. Steins. 



i. Primary structure. 



In a transverse section of a young Sphenophyllum stem such 

 as that diagrammatically sketched in fig. 105, A, we find in the 

 centre the xylem portion of a single stele with a characteristic 

 triangular form. The primary xylem consists mainly of fairly 

 large tracheae with numerous pits on their walls ; towards the 

 end of each arm the tracheids become scalariform, and at the 

 apex there is a group of narrower spiral protoxylem elements. 

 In the British species there is a single protoxylem group at the 

 apex of each arm, but Renault has described some French 

 stems in which the stele appears to be hexarch, having two pro- 

 toxylem groups at the end of each of the three rays of the stele. 

 The primary xylem strand of Sphenophyllum has therefore a 



> e.g. Newberry (91). ^ Benault (73), (W), (96). 



3 WiUiamson (74), (78). ^ Williamson and Scott (94), p. 919. 



