350 Causes and Course of Organic Evolution 



character of being for a time monostelic {121: 95). That is, 

 in the midst of the differentiating stem tissue a central core 

 of vascnhir elements matures into wood or xvlem, and bast 

 or phloem, constituents. With advancing age and growth of 

 the stem, the axial core divides, so that a circle of several 

 bundles results. 



The leaves are at first simple lateral lobes of the axis, but 

 become increasingly enlarged as new ones arise, till expanded 

 simple or lobed floating leaves are formed. In the variety 

 flidtans, that the writer has kept under observation for years 

 in tanks at the Botanic Garden of the University of Penn- 

 sylvania, such leaves or only deeply pinnatifid leaves are pro- 

 duced. Abundant buds, that propagate the plant asexually, 

 arise in the sinuses of these. 



But, in the normal form that advances to a semi-aquatic 

 state, greatly more complex and finely divided leaves arise, 

 that bear simple scattered subglobular sporangia. The simple 

 but variable annulus with its abundant closely-placed cells, 

 the short broad stalk, and the few spores produced within — 

 usually 16 — are all suggestive of a more primitive type than 

 is seen in any other existing fern. We accordingly place it 

 at the base of the fern series, for, though it may have under- 

 gone a degree of simplification by exposure from changing 

 subaquatic to aquatic conditions, the steady advance shown 

 in the life history of each individual militates fairly against 

 carrying such a view too far. 



But between a Ceratopteris type and the highest chseto- 

 phoraceous algae a wide gap exists, that at present we can 

 only fill in by hypothetical examples based on combined views 

 obtained from living forms. Thus by reference to Figure 13 

 (p. 356) it will be seen that between the Chsetophoracese and 

 Ceratopteris a wide stretch of life evolution must have pro- 

 ceeded during which the great equisetal, cordaital, and pos- 

 sibly the cycado-filicineal alliances may have become some- 

 what established. 



Now these all still clearly indicate that they had a tludloid 

 or algoid origin, in that they show in common a purely cellular 



