290 EVOLUTIONARY BEARINGS OF THE RESULTS [CH. 



Consequently a critical point will alwa3^s be coming nearer, and may ulti- 

 mately be reached, where the surface area will be functionally insufficient. 

 All Ferns begin their individual development with a small, solid, vascular 

 tract: it enlarges conically upwards as the plant produces successively larger 

 leaves. In the absence of any secondary thickening, each plant is thus con- 

 stantly approaching that limit of size when further development would be 

 functionally impracticable. The difficulty can be overcome by change in 

 form of the stelar column. Any change of form from the cylinder or cone 

 would give an increased proportion of surface to bulk. Such changes are 

 actually illustrated by comparison of sections of the stele in certain allied 

 fossil stems of different sizes, all drawn to the same scale (Fig. 175). The 

 smallest steles are approximately cylindrical : the larger steles become fluted, 

 or in transverse section stellate ; the involutions being deepest where the size 

 is greatest. The change in form certainly does result in an increase in the 

 proportion of surface to bulk, beyond what the proportion would have been 

 if the enlargement went along with a simple conical form. Results similar in 

 principle though different in detail may be seen in all the larger modern 

 Ferns (Vol. I, Chapter X). Successive sections from the same individual 

 Fern-stem, as it enlarges upwards, serve to illustrate the changes which 

 affect the proportion of surface to bulk of the vascular tracts (Fig. 178). 

 Similar reactions appear in many other plants and plant-parts, provided that 

 secondary thickening does not step in and vitiate the problem. In fact 

 the principle is of wide application. In the long run it is held to account 

 causally for that curious disintegration of the vascular system which 

 is so marked a feature in the larger, and particularly the more advanced, 

 Ferns. 



The form of the vascular system is so far distinctive and constant in 

 character that it is made use of ever more and more in the systematic com- 

 parison of Ferns (Chapters VII-x). Its features have become hereditary, 

 though subject to variability in detail. A protostelic Fern, such as Gleichenia 

 or Cheiropleuria, remains protostelic: a solenostelic species or genus, such 

 as Loxsonia, is constantly solenostelic : a polycyclic type, such as Matonia, 

 regularly becomes polycyclic as it matures to the adult state : every sporeling 

 of a typically dictyostelic Fern assumes that character as it develops. The 

 relation between the form of the vascular tracts and the limiting factor of 

 size may in each be clearly traced, though the necessary adjustment may 

 differ in detail from one phylum or genus to another: but in point of fact 

 those adjustments do become within limits fixed, and are transmitted as 

 such in descent. Here again a viodification, primarily of the nature of an 

 ontogenetic adfnstnient, has become an accentuated and inlicrited feature in the 

 more advanced Ferns, and has even acquired high diagnostic value in the hands 

 of the systematist. 



