364 PART IV. CLASSIFICATION. 



The sporangium may be sessile (Gleicheniaceae, most Schizseacese. 

 Hymenophyllacese) ; or shortly stalked (Lygodium, Cyatheacete, 

 Osmundacese * ; or it may have a usually rather long slender stalk 

 consisting of two or three longitudinal rows of cells (Polypodiacese) ; 

 this is dependent upon whether the originally-formed stalk-cell 

 developes further or not. 



The spores are set free by the dehiscence of the sporangium ; 

 this takes place at a certain part which, though different in the 

 various forms of sporangia, is always closely connected with the 

 annulus and is termed the stomium (see Fig. 215) ; dehiscence 

 begins by a split between (not through) the cells of the stomium. 

 In the Polypodiaceae the plane of dehiscence is at right angles to 

 the long axis of the sporangium, and the stomium is situated on 

 the margin between the end of the annulus and the stalk. 



A striking feature in the general morphology of these plants is 

 the presence on the stem and the bases of the leaf-stalks, especially 

 when young, of numerous scaly hairs (ramenta or palece), which 

 consist usually of a single layer of cells, with more or less thickened 

 brown walls ; they are of various shape, and frequently have 

 marginal glandular hairs secreting tannin or mucilage, the latter 

 generally in the neighbourhood of a growing-point or stem or leaf. 

 Less commonly, glandular hairs are developed on the leaves, as in 

 species of Gymnogramme (Gold and Silver Ferns), in which the 

 under surface of the leaves is covered with a yellowish dust, 

 consisting of minute needles of resinous and waxy substances, 

 secreted by the hairs. Root-hairs occur on subterranean stems 

 and leaf-stalks. 



A primary root is developed, probably in all forms, but in no 

 case does it persist in the adult. In the full-grown plant all the 

 roots are adventitious; they spring in great numbers from the 

 stem or the leaf-stalks. The roots are small and branched ; the 

 branching is lateral, and the growing-points of the young roots 

 are developed each from a single cell of the endodermis, termed 

 a rhizogenic cell, situated opposite to a xylem-bundle of the 

 central cylinder. In most cases the growing-point of the root 

 has a single pyramidal apical cell (see Fig. 86) with three flat 

 sides and a spherical base directed outwards. 



Adventitious buds, subserving vegetative propagation, are com- 

 monly produced ; they arise most frequently on the subterranean 

 portions of leaf-stalks (as inPteris aquilina, Aspidium Fili.i--mas\ 

 and sometimes, as in Onoclea Struthiopteris, the bud grows into 



