404 SPHENOPHYLLUM. [CH. 



A and B. The wood consists of pitted tracheae, with two groups 

 of protoxylem elements at each of the truncated angles of the 

 solid strand of xylem. From the angles of the stele branches 

 of vascular tissue pass out through the cortex to supply the 

 sterile and fertile segments of each verticil. One of the 

 transverse sections of the Sphenophyllum cone in the British 

 Museum Collection (no. 1898 E) affords a good example of the 

 misleading appearance occasionally presented by an intruded 

 ' rootlet ' of Stigmaria ; the vascular tissue of the cone has 

 disappeared, and a Stigmarian appendage with its vascular 

 bundle occupies the position of the stelar tissues. 



The bracts consist of parenchymatous tissue limited exter- 

 nally by an epidermis containing stomata. A single stoma 

 with subsidiary cells is represented in fig. 107, A. The 

 sporangiophores are composed internally of thin -walled cells 

 with stronger cells towards the surface. The longer sporang- 

 iophores in a series may be more or less coherent for part of 

 their length to the upper surface of the verticil of bracts. In 

 fig. 108 the slender sporangiophores do not appear to come 

 off always from the same portion of the bracts, but this is due 

 to some of them lying on the surface of the latter during part 

 of their course to support the external circle of sporangia. 

 The hook-like distal end of a sporangiophore, towards the point 

 of attachment of the sporangium, is characterised by the larger 

 size and greater prominence of the surface cells ; these larger 

 cells, which pass over the upper surface of a sporangium base, 

 probably constitute a kind of annulus which determines the 

 dehiscence of the sporangial wall'. 



Fig. 107, G, represents a sporangiophore and its sporangium 

 cut through transversely just below the point of attachment of 

 the latter to the end of the hook-like termination of the former. 

 The spores are characterised by an irregularly reticulate 

 thickening of the outer coat or exospore, as seen in the figure. 



One of the chief points of interest suggested by a Spheno- 

 phyllum cone is the exact morphological nature of the sporang- 

 iophores. Are they branches borne in the axils of bracts, or 



1 For a more complete account of this strobilus vide Zeiller (93), and 

 Williamson (W), etc. 



