XXXIII] 



ANATOMY 



299 



(Fig. 562). At the base of the stem there is a protostele with a solid core of xylem, from 

 which the first leaf-trace departs without disturbance, but the phloem is prolonged down- 

 wards in its axil. At the departure of the subsequent leaves 

 this prolongation becomes more pronounced ; and sub- 

 sequent steps follow in a manner similar to what has been 

 seen in the ontogeny of G. pectitiata (compare Fig. 481, 

 also Vol. I, Fig. 134), with the result that at the level 

 of about the eighth leaf the stem is solenostelic. This 

 stage does not last long in Alsophila, for leaf-gaps 

 overlap, and the system gradually becomes dictyostelic. 

 The leaf-trace of the first five or six leaves is undivided, but 

 later two or three strands and at the tenth leaf four strands 

 pass into each leaf-base, two from each side of the leaf-gap. 

 Thus the sporeling of Alsophila passes through similar 

 stages tothose of Gleicheiiiapectinata, butmoreabbreviated, 

 and with a divided leaf-trace (compare Vol. i, Fig. 134, p. 

 144). The first indication of the internal medullary system 

 in Alsophila was found by Gwynne-Vaughan at about the 

 tenth leaf. Just below the upper (adaxial) traces of this 

 leaf the xylem projects inwards forming a small ridge: 

 sometimes it separates as a small xylem-strand lying free 

 within the phloem, ending blindly or fusing up again with 

 the main xylem. Later such strands may separate from the 

 main meristele, running as independent vascular strands, 

 and initiating the medullary system, which thus owes its 

 origin to a local thickening at the margins of the leaf-gaps : 

 but the medullary strands do not appear at all till the 

 ordinary cylinder has become dictyostelic. 



The ontogeny thus disclosed for a complex Tree-Fern 

 may be held as a valid suggestion of the way in which the 

 adult condition arrived in descent. It starts from a protostelic 

 state which quickly passes to the solenostelic, and this again 

 to the dictyostelic : lastly by intrusion from the margin of 

 the leaf-gaps the medullary system is produced. All of these 

 are probable steps in the evolutionary story of plants with 

 a massive axis bearing large and closely disposed leaves. 

 The several steps are represented by the protostelic Glei- 

 chenias, by the solenostelic G. peciinata, and hy Lophosoria 

 with its transition to dictyostely. The stelar ontogeny thus upholds the sequence suggested. 



Fig. 562. Alsophila exceha. Dia- 

 gram of the vascular system of 

 a young plant in median longi- 

 tudinal section. The xylem is 

 black, the phloem highly 

 shaded, and the endodermis 

 is indicated by a dotted line : 

 the ground-tissue is left white. 

 (After Gwynne-Vaughan.) 



SoRi AND Sporangia 

 The three genera are distinguished from one another by the character of 

 the indusium, the sori being constructed otherwise on the same plan in them 

 all. Their disposition on the leaf or segment is fundamentally the same as 

 in Gleichenia, a single series ranging on either side of the midrib. As a rule 

 only one sorus is borne on each vein, but the position relatively to the margin 

 varies, being sometimes near to the midrib, as in most of the highly pinnate 

 types, e.g. CyatJica serra : sometimes they lie far out towards the margin, as 



