XIX] 



ANATOMY 



57 



that more complex type. But the fact that the monarch structure may 

 appear at the very base of insertion both in BotrycIiiiDii and in O.phioglossum 

 presents a difficulty in accepting this. 



TJie Stem 



All the Ophioglossaceae have a definite stele in the basal region of the 

 axis delimited by an endodermis, which is, however, of a rather rudimentary 

 type. In the ontogeny of Pteridophytes a coherent body of tissue called the 

 stele, partly made up of elements having a truly cauline origin, exists from 

 the first, and it serves to connect up adjacent leaf-traces. Where the axis 

 preponderates over the leaf this composite nature of the stele is readily seen 

 at an early stage of the ontogeny, as in Lycopods. But where as in the 



Fig. 349. A, B, transverse sections of the young stem of Botrychiuin Ltinaria, showing medullation, 

 the departure of the leaf-trace, and the first steps of cambial activity. Note isolated tracheides in 

 the pith of .J, and the complete endodermal investment of i?. ( x 125.) 



Ferns the leaf preponderates over the axis in the young shoot, the stage 

 when this composite nature can be recognised will appear later (see Vol. I, 

 Chap. Vir, p. 139). It happens that the Ophioglossaceae in their extreme 

 types, and doubtless as a consequence of their monophyllous habit, present 

 peculiar difficulties in this recognition. These are, however, less serious in 

 Botrychiuin and Helminthostachys, in which the cauline factor is stronger, 

 than in Ophioglossum where it is peculiarly weak. The whole question is 

 closely related to that of the constitution of the shoot itself, and the relation 

 of axis and leaf not only structurally but also in time of origin. The view 

 adopted here is that the two constituents of the shoot co-exist from the first, 

 as indeed embryology clearly demonstrates in all but extreme instances: 

 but that the balance of their importance in the shoot may vary. 



