GROUP III. PTERIDOPHYTA. 373 



organ, no trace of which persists in the adult. It is'the organ of 

 attachment of the embryo-sporophyte to the gametophyte ; and it 

 is also the absorbent organ by which the embryo, until it is able 

 to absorb and assimilate food for itself, obtains its nourishment 

 from the prothallium (compare Bryophyta, p. 330). 



The development of a suspensor in the Lycopodinae is an adap- 

 tation correlated with the fact that the nourishment of the 

 embryo in that group depends upon its coming into direct contact 

 with the tissue of the massive gametophyte, the cells of which 

 are filled with nutritive substances. 



A primary root, that is, a root developed from the hypobasal half 

 of the oospore, and so situated at its origin that its growing-point 

 is in a straight line with that of the stem (see p. 15), only 

 occurs in the Filicinae and Equisetinae ; but even here it does 

 not persist as a tap-root in the adult: in these plants numerous 

 adventitious roots are developed. In the Lycopodinae, where 

 there is no primary root, all the roots are adventitious. 



Some adult forms are altogether without roots : as Salvinia, 

 and some species of Trichomanes, among Filicinae ; Psilotum and 

 Tmesipteris, among Lycopodinae. The functions of the root are 

 discharged, in Salvinia by modified leaves, in the others by 

 modified branches. In the absence of information as to the 

 embryogeny of these rootless plants, except Salvinia, it is not 

 possible to state definitely that they are, like Salvinia, rootless from 

 the first: for it is conceivable that they may have a short-lived 

 primary root which entirely disappears as the embryo developes. 



The branching of the root is generally lateral in the Filicinae 

 and Equisetinae ; it is dichotomous in the Lycopodinae and in 

 Isoetes. In the former case, the lateral rootlets are developed, 

 in the Filicinae, from cells (rhizogenic) of the endodermis which 

 are opposite to the xylem-bundles of the stele ; in the Equisetinae, 

 from the cells forming the inner layer of the two-layered endo- 

 dermis. 



The stem is generally short and unbranched in the Filicinae ; 

 generally elongated and much branched in the Equisetinae and 

 Lycopodinae. 



The leaves are differentiated into foliage-leaves and sporophylls 

 in the Equisetinae and generally in the Lycopodinae, but not in the 

 Filicinae as a rule : the foliage-leaves are relatively large in 

 proportion to the stem in the Filicinae, relatively small in the 

 Lycopodinae, reduced to cataphylls in the Equisetinae. 



