212 



Commercial Gardening 



generally which has enabled the botanists to classify and divide Ferns into 

 a large number of distinct genera, determined by such arrangements, while 

 these genera are again divided into species, which all have the same form 

 of fructification, but vary in make, habit, and other characters on more or 

 less definite lines. In the vast majority of cases the spores are arranged 

 on the backs of the fronds in isolated dots or roundish heaps (Poly podium) 

 (fig. 305 A), in definite lines along the veins within the margin (Asplenium) 

 (fig. 305 D), in continuous or interrupted lines along the margin (Pteris, 

 fig. 305 E, Scolopendrium, Adiantum, fig. 305 F), in continuous even layers 

 covering the surface (Platycerium); while in some genera, such as Osmunda, 



Fig. 305. Showing Pinnules and Sori of various genera of Ferns 



A, Polypodium. B, Aspidium. C, Nephrodium. D, Asplenium. E, Pteris. F, Adiantum. 

 G, Sorus of Hymenophyllum 



Anemia, and others, the fructification is confined to definite portions of the 

 frond, or as in Struthiopteris and Onoclea to entirely specialized fronds, 

 a more or less clustered form being adopted at the expense of the leafy 

 development. Apart from these and many other types of arrangement, 

 characterizing different genera, the groups of sporangia are differentiated 

 by the presence or absence of a protecting cover called the indusium 

 (fig. 304 i), and the particular form which this cover assumes. Thus we 

 have three native genera both characterized by bearing the sori or spore 

 heaps in a round isolated form, viz. Polypodium, Polystichum, and Nephro- 

 dium (Lastrea). The Polypodium, of which we may take one common 

 species, P. vulgare, or the exotic P. glaucum as types, has these heaps 

 entirely destitute of a cover, as, of course, have all its generic relatives. 

 The other two, on the other hand, have distinct semi-transparent scale- 

 like coverings or indusia, but are differentiated by those of Polystichum 



