LINKEAN SOCIETY OF LONDON. 71 



They were originally stated in bis paper on the Sphenophyllale* 

 and Equisetales, published in l'J03 *, and have been more fully 

 developed in his recent essay on the Morphological Evolution of 

 the Vegetable Kingdom (]908)t. Prof. Liguier is a strong 

 advocate of the homologous theory of alternating generations. 

 He derives all tlie higher plants (Bryophyta and Yusculares) from 

 a hypothetical group, his Prohepatics, in which the hfe-cycle 

 embraced two phases — sporophyte and gametophyte, — which, he 

 suggests, probably also existed in their marine ancestors, as is the 

 case in tlie recent Dlcfuota. He supposes that in the Prohepatics 

 the sexual and asexual individuals had a flat dichotonious thallus, 

 and that they were a(]uatic or semi-aquatic in habit. Typical 

 antheridia and archegonia may have already appeared at this 

 early stage; the author refers to their long persistence through 

 so many of the higher groups — an example, as Prof. Bailer has 

 pointed out, of morphological stability dependent on adaptation. 

 From the Prohepatic ancestors evolution proceeded in two main 

 directions. In the Bryophyta the gametophvtic phase assumed 

 the greater importance, vegetatively, while the sporophyte became 

 reduced in relation to its parasitic lite. In the line of the 

 Yasculares the reverse process went on : the gametophyte under- 

 went progressive and ultimately extreme reduction, while the 

 sporophytic phase showed an immense advance in vegetative 

 organization, some of the branches of the thallus (cauloids) 

 becoming specialized as organs of absorption, i. e. roots, which 

 are acutely described as simply the. most ancient of rhizomes. 

 He cites the Lycopods as still showing traces of the common origin 

 of aerial cauloids and roots. 



Concurrently with this important differentiation of absorptive 

 organs proceeded the no less momentous differentiation of an 

 internal conducting system, by which these plants became 

 vascular, and thus fully adapted to a terrestrial life. 



As the sporophyte thus attained a more and more exuberant 

 growth, a new differentiation of its aerial parts set in. The 

 original dichotonious branching of the thallus became, as Prof. 

 Lignier puts it, " sympodised," certain branches becoming pre- 

 dominant, and thus forming an (Lvis, while others were subordinated 

 and constituted the beginnings of leaves. Here, however, a great 

 distinction manifested itself, on which the main divisions of 

 Prof. Lignier's system are based. In the Lycopodiaceae the 

 sympodisation of the thallus never went very far, and true leaves 

 (limited thallus-branches) \\ere never developed. The small 

 leaves characteristic of the Lycopods — "phylloids," as our author 

 calls them — are not thallus-branches at all, but have always 

 been of the nature of appendages and are derived from lamellar 

 hairs already present at the Prohepatic stage. They are 



* " Les Equisetales et Splienophyllales," Bull. Soc. Linu. de Noniiaiulie, 

 ser. 5, t. vii. p. 9o. Caen, lOOo. 



t Cotiiptes Eendus de I'Assoc. Franc;, pour rAvaucenient des Sci. 1908, 

 p. 530. 



