202 



DIPTEROID FERNS 



[CH. 



(Fig. 711, A, B). The narrower fertile leaf is unbranched: its lower surface 

 is covered by a dense mass of sporangia, which will call for more special 

 description later. 



The hairs of Cheij-opleuria conform more nearly to those of Matonia than 

 of Dipteris, in both of which they are simple, and indurated at the distal 

 end, while the basal cells retain the power of growth and division. Here each 

 hair is long and unbranched, and is composed of some 20 or more thin-walled 

 cells: in fact it is of a more primitive type than in either of these genera. 



The vascular anatomy is of outstanding interest, for the axis is protostelic 

 (Fig. 712, /^), the stele resembling in structure and in form that of Gleichenia. 



Fig. 711. ^ = a juvenile leaf of Cheiropleuria showing the venation, 

 for comparison with that of Dipteris conjtigata. Vol. n, Fig. 570. ( x 3.) 

 B = z. juvenile leaf of PlatyceriiiDi Veitchii, for comparison with these. 

 (X3-) 



Obliquely from the upper side of it the leaf-traces pass off alternately right 

 and left, their origin being as follows. First, a group of protoxylem-tracheides 

 appears, some 3 or 4 layers within the outer limit of the metaxylem, while 

 its contour swells into a rounded hump. Parenchyma-cells then aggregate 

 internally to this, and a loop of xylem is thus formed, with the protoxylem 

 lying centrally within it (Fig. 712, A). This, together with some of the 

 internally-lying tracheides, moves outwards, a lateral constriction appearing 

 on each side, till the leaf-trace is shut off from the stele by the intruding 

 phloem and sheaths (Fig. 712, E). On separation it consists of an oval tract 

 of tissue, enclosing within a ring of metaxylem a parenchymatous island, 

 with the protoxylem (which has meanwhile divided into two strands) at its 



