690 CONCLUSION 



evolution of the race, the evidence is quite clear : it indicates that the 

 large-leaved forms, in which solenostelic or dictyostelic structure rules, 

 originated from a smaller-leaved ancestry, with protostelic structure and a 

 single strand of the leaf-trace. This is in full accord with probability, 

 according to the antithetic theory of origin of the leafy sporophyte ; for on 

 that theory smaller-leaved would necessarily have preceded larger-leaved 

 types. 1 



Another mode of amplification of the stele, which often accompanies 

 the first but is not necessarily associated with it, is by secondary thickening. 

 The stem of Sphenophyllum (Fig. 217), and of Ltpidodendron Petticurensis? 

 are examples of how a secondary development of vascular tissue may 

 surround a solid protostele : this shows that medullation does not neces- 

 sarily precede secondary thickening, but commonly the secondary thickening 

 occurs where medullation is present : and indeed in some cases the two 

 are in a sense complimentary, the secondary vascular tissue taking the 

 place functionally of the primary tissue reduced by medullation ; this is 

 exemplified in the Calamarians (Fig. 225) and in Sigillaria? as also in 

 some forms of Stigmariaf and it is seen with special clearness in Lygino- 

 dendron, Poroxylon, etc. In other types structurally more advanced, the 

 secondary development may be held to have completely replaced the 

 centripetal wood of the original stele. 



The distribution of secondary vascular development among the Pteri- 

 dophyta indicates clearly that it is a phyletic afterthought, originated in 

 relation to the increasing size of the vegetative system consequent upon 

 continued apical growth, repeated branching, and leaf-enlargement, either 

 separate or in combination. Enlargement of the primary stele, with or 

 without attendant medullation, may meet the demand in some degree ; 

 but it is a fixed and limited scheme compared with that of secondary 

 thickening, which is capable of increasing the conducting tract in proportion 

 to the demand. In some cases, however, it appears that a phyletic decrease 

 of the secondary development has occurred, and it is probable that the 

 feeble cambial activity in the nodes of Equisetum, and locally in the 

 Psilotaceae may be vestigial remains of a more active increase in their 

 predecessors, allied respectively to the ancient Calamarians and Sphenophylls. 



1 This is, however, quite contrary to the opinions of Dr. Jeffrey, who holds that the 

 large-leaved and small-leaved stocks were "separate back to the beginning of the period 

 when the palaeontological record begins." This view would recognise no transition from 

 the structure characteristic of the smaller-leaved forms (cladosiphonic) to that characteristic 

 of the larger-leaved (phyllosiphonic). But, as a matter of fact, this can be demonstrated 

 to have occurred in the individual life of Ferns, and probably it has occurred also in other 

 forms in the passage from small-leaved youth to large-leaved maturity. It has been 

 pointed out repeatedly in Part II. how cladosiphony is the anatomical expression of the 

 dominance of axis, phyllosiphony that of the leaf in the shoot : and the balance may 

 be altered in the individual life. (See Jeffrey, Phil. Trans., 1902, vol. 195, p. 144.) 



2 Kidston, Proc. Roy. Soc., Edin., 1906-7, p. 208. 



3 Scott, Studies, Figs. 77-78. 4 /., p. 234. 



