CYATHEAE 609 



with the main stele, except sometimes by a small branch near their point 

 of origin. 



It seems, therefore, that the internal vascular strands of Alsophila 

 excelsa owe their existence to the same initial phenomena as do those of 

 Dennstacdtia rubiginosa ; that is to say, they are probably derived from 

 the elaboration of a local thickening of the xylem-ring at the margins of 

 the leaf-gaps in the ordinary stelar cylinder ; but they do not appear at 

 all until the ordinary stelar 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 mature condition was achieved 

 in descent. It starts from a protostelic state, which is, however, brief, and 

 passes to the solenostelic by intrusion of outer-lying tissues into the xylem- 

 core ; but this again passes into the dictyostelic by reason of the overlapping 

 of the leaf-gaps : and lastly, by intrusion of vascular growths from the 

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

 steps, so quickly passed over in the individual life, are readily intelligible, 

 and even probable, in the evolutionary story of plants with a massive 

 axis, bearing large and closely disposed leaves. 



The protostelic state, here so short, is the permanent condition in 

 most of the Gleicheniaceae. But the most advanced species of Gldchcnia 

 (G. dichotoma and pectinata) show signs of solenostely, while in Alsophila 

 pritinata the solenostelic state appears to be permanent. But in other 

 species of Alsophila it also is a phase quickly passed through to the 

 dictyostelic state, which is then permanent. Finally, the medullary 

 system absent in A. pruinata, as it is also in Dicksonia, but developed 

 in Cyathea, is clearly a late accessory, probably consequent upon the 

 enormous distension of the pith in relation to the wide leaf-bases. 



The leaf-trace also presents features of comparative interest : in the 

 young plant it consists of a single strand, as it is in all the Gleichenias : 

 in A. pruinata it appears to be so at the base even of the mature leaf, 

 though it soon breaks up into separate strands as it passes up into the 

 leaf-stalk : but in most of the Cyatheae the leaf-trace in the mature 

 shoot is from the first composed of a number of distinct strands. These 

 successive steps again indicate a probable phyletic progression, the young 

 plant showing a condition similar to that seen in simpler types, and 

 especially in the Gleicheniaceae. 



If the facts derived from the characters of the sorus be put into 

 relation to these from anatomy, a substantial parallelism emerges, point- 

 ing in both cases towards the Gleicheniaceae as a probable indication 

 of the genetic source. In soral characters Alsophila is the nearest to 

 Glcichenia, and especially to those species in which the sorus is no 

 longer uniseriate, but consists of a large number of relatively small 

 sporangia (G. dichotoma and pcctinata'). It is in these very species- that 

 there is a definite advance towards a state of solenostely not very far 



removed from that actually seen in A. pruinata. From such a 



2 Q 



