60 THE MORPHOLOGY OF PTERIDOPHYTES 



in some species). They are provided with a root cap and 

 their root-hairs are paired (a most pecuHar arrangement). 

 The majority are diarch with a crescent-shaped xylem area, 

 but in some species the stele is very similar to that of the 

 stem, as in Lycopodium clavatum, where the xylem takes the 

 form of parallel plates. 



Variations from species to species in the shape of the 

 sporophylls have already been described. In addition, there 

 is considerable variation in the manner in which the 

 sporangium is borne in relation to the sporophyll. In some, 

 e.g. Lycopodium selago and L. inundatum, the sporangium 

 is in the angle between the sporophyll and the cone axis, i.e. 

 it is axillary. In others, e.g. L. cernuum and L. clavatum (Fig. 

 loC), the sporangium is borne on the adaxial surface of the 

 sporophyll and may be described as *epiphyllous'. The 

 sporangial initials arise at a very early stage in the ontogeny 

 of the strobilus, normally on the ventral side of the sporo- 

 phyll, but in some species actually on the axis, whence they 

 are carried by subsequent growth changes into the axil. The 

 first sign of sporangial initials is the occurrence of perichnal 

 divisions in a transverse row of cells (three to twelve in 

 number) (Fig. loA). The innermost daughter cells provide 

 the archesporial cells by further division and also contribute 

 to the stalk of the sporangium, while the outermost cells (the 

 jacket initials) give rise to the wall of the sporangium (Fig. 

 loB). This is three cells thick just before maturity, but then 

 the innermost of the layers breaks down to form a tapetal 

 fluid. Like the sterile leaves, the sporophyll has a single vein, 

 which passes straight out into the lamina, leaving the 

 sporangium without any direct vascular supply. The mature 

 sporangium is kidney-shaped and dehisces along a trans- 

 verse line of thin-walled cells, so liberating the very numerous 

 and minute spores into the air. 



In some species, the spores germinate without delay, while 

 still on the surface of the ground, but in others there may be 

 a delay of many years, by which time they may have become 



