70 THE MORPHOLOGY OF PTERIDOPHYTES 



The primary cortex was relatively thin-walled and, within 

 it, a number of different regions are recognizable. Of these, 

 the most interesting is the so-called 'secretory tissue', made 

 up of wide thin-walled cells, whose horizontal walls became 

 absorbed in the formation of longitudinal ducts. Each leaf- 

 trace, as it passed through this region, acquired a strand of 

 similar tissue which ran parallel with it before splitting 

 into two 'parichnos strands' on entering the base of the leaf. 

 It is beheved that the secretory tissue was in some way 

 connected with the aeration of the underground organs, pro- 

 viding an air path from the stomata of the leaves, through 

 the mesophyll to the parichnos strand and so to the secretory 

 zone, which was continuous with a similar region in the 

 cortex of the Stigmarian axes. 



The leaves, known as Lepidophyllum, were borne in a spiral 

 with an angle of divergence corresponding to some very 

 high Fibonacci fraction such as -fir^, -Us, etc. They 

 were Hnear, up to 20 cm long, triangular in cross-section 

 and with stomata in two longitudinal grooves on the 

 adaxial side. The vascular strand remained unbranched as 

 it ran the length of the leaf. The leaves were shed from the 

 trunk and larger branches by means of an absciss layer, and 

 the shape of the remaining leaf base and scar provides im- 

 portant details for distinguishing the various genera and 

 species. Fig. 12B shows the appearance of the trunk of a 

 Lepidodendron where, characteristically, the leaf bases were 

 elongated vertically. In some species, the leaf bases became 

 separated shghtly as the trunk increased in diameter, but in 

 others they remained contiguous, even on the largest axes. 

 No doubt this was brought about to some extent by an 

 increase in the size of the leaf base, much as a leaf scar 

 becomes enlarged on the bark of many angiospermous trees, 

 but such increase must have been relatively slight, for other- 

 wise the leaf bases would have become much broader in 

 proportion to their height. Evidently, therefore, the largest 

 leaf bases must have been large from the start, from which 



