340 



ORGANOGRAPHIC COMPARISONS 



[CH. 



It is not even suggested that the Psilophytales include the direct ancestry 

 of the FiHcales. The intention is only to indicate the general similarity which 

 the verbal sketch bears to these ancient ex- 

 amples of land-vegetation. The chief points 

 of difference lie in the higher differentiation of 

 the shoot shown by Ferns, and the establish- 

 ment of a root-system. The possible origin of 

 the Fern-shoot from a simpler source was fore- 

 casted from a comparative study of Ferns and 

 Cycads in 1884 (Bower, P]iil. Trans. 1884, 

 Part II, p. 605). It was asked, "May we not 

 with good reason think that, just as the phyllo- 

 podium gradually asserts itself as a supporting 

 organ among parts originally of similar origin 

 and structure to itself, so also the stem may 

 have gradually acquired its characters by differ- 

 entiation of itself as a supporting organ from 

 other members originally similar to itself in 

 origin and development? Thus the stem and 

 leaf would have originated simultaneously by 

 differentiation of a uniform branch-system into 

 members of two categories, and this is what is 

 actually illustrated in the case of the phyllo- 

 podium and pinnae in the series of plants above 

 discussed." This suggestion was "left in the 

 air," owing to the entire absence at the time 

 when it was written of any direct evidence to 

 support it, such as might be supplied by more 

 primitive vascular plants. Now with the Psilophytales before us, and with 

 the striking similarity of vascular structure between the leaf and the axis 

 oi Botryoptcris cylindrica, together with other anatomical evidence now avail- 

 able, the idea of the differentiation of axis and leaf as seen in the Filicales 

 from an indifferent dichotomising system emerges as a reasonably tenable 

 hypothesis. 



It has been suggested in the final paragraph of Chapter XV, p. 3 1 5, on Em- 

 bryology, that as the first leaf and the axis originate jointly from the epibasal 

 hemisphere in all Ferns, the primitive spindle may have had the power of 

 branching at its apex, and that these two parts were the result. It has lately 

 been shown by Holloway that the embryo of Tinesipteris actually does this 

 occasionally, and that the branches of the indifferent " shoot-region " may 



tniffAriyM^ 



Fig. 308. Hornea Ligiiteri, recon- 

 struction by Kidston and Lang; 

 quoted here as an example of a 

 very early type of land-living 

 sporophyte. 



be equal, or in various degrees unequal. The former is sh( 



Fig. 309. 



Here the epibasal region has divided into two equal parts, each endowed 



