390 PART III. THE CLASSIFICATION OF PLANTS. 



The leaf arises from a single superficial cell of the growing- 

 point of the stem. When young it is strongly hyponastic (see 

 p. 60), so that, as it elongates and branches, both the main axis 

 of the leaf (phyllopodium) and the lateral branches become 

 inrolled upon themselves like a crosier : in other words, the 

 vernation is circinate: as it grows older the growth becomes 

 epinastic, and thus the leaf becomes expanded. 



In the great majority of these Ferns the sporophylls are simply 

 foliage-leaves bearing sporangia on the dorsal surface, but in 

 certain cases they are more or less specialised. Thus, in Onoclea 

 Struthiopteris, the sporophylls are smaller than the foliage-leaves, 

 and have narrower pinnae : in the Hard Fern, Blechnum boreale, 

 the sporophylls are longer and have narrower pinnae than the 

 foliage-leaves: in Osmunda regalis the pinnae of the upper branches 

 of the sporophyll are reduced to little more than the midrib, the 

 pinnules being represented by clusters of sporangia : in Aneimia 

 (e.g. A. Phyllitidis') generally the lowest pair of pinnae of the 

 sporophyll alone bear sporangia; these pinnae consist merely of the 

 nervature bearing numerous sporangia, and are erect on much 

 elongated secondary petioles : in Platycerium alcicorne there is a 

 curious instance of heterophylly : the foliage-leaves are broad, 

 and closely appressed to the substratum, whereas the sporophylls 

 are erect, narrow, and branched. 



The sporangia are but rarely borne on the superior (ventral) 

 surface of the sporophyll (e.g. OJfersia cervina) ; more commonly 

 on the margin (e.'j. Hymenophyllacese, Dicksonia, Davallia) ; but 

 as a rule, on the dorsal surface, either near to the margin (e.g. 

 Pteris, Adiantum), or distant from it (e.g. Asplenium, Aspidium, 

 etc). They are usually developed in connexion with the nervature 

 of the sporophyll, but sometimes also from the intervening tissue 

 of the lamina (Acrosticheae, such as Polybotrya, Chrysodium) : in 

 the former case they occur in groups, termed sori, which are 

 commonly isolated, but occasionally (e.g. Pteris) a continuous 

 marginal sorus is formed. 



The sorus generally consists of a large number of sporangia : in 

 the Gleicheniaceae, however, the number is small (2-8); and in 

 some cases (e.g. Lygodium) there is only a single sporangium. 

 The sporangia of the sorus are borne on a projection of tissue, 

 the placenta or receptacle, which presents various forms : it may 

 be a slight rounded elevation (e.g. Aspidium) ; or more elongated 

 and conical (e.g. Cyathea, Hymenophyllum) ; or very long and 



