June 1903.] BOTANICAL SOCIETY OF EDINBUKGH 313 



I have not the intention of entering here on a detailed 

 description of fern fructification and early development; of 

 the prothallus and the alternation of generations, on which 

 so much has been written and might here be said, but, as 

 it is the case that no fewer than forty of our forty-seven 

 native species fall naturally into one tribe, the Polypodiece 

 (the remaining seven being peculiar, and grouped in three 

 small tribes), a few remarks regarding the fructification of 

 the Folyipodiece may be appropriate. 



Fern spores, as said, are microscopic and beyond the 

 unassisted eye to see. In the aggregate they form the 

 fern-dust of imaginative writers. They are not naked and 

 exposed to the atmosphere, but are contained in minute, 

 long-stalked capsules (capable, as is mentioned by Mr. 

 G. F. Scott Elliot, r,L.S., in his recent book, " Nature 

 Studies," of holding sixty-four spores), and known 

 each as a sijorangium, which bursts when ripe by 

 the rupture of the annulus surrounding it, so shedding 

 the spores. These siwrangia, or spore-cases, are de- 

 veloped on the under surface of the fern frond, 

 and they form, grouped together, the circular patches 

 known each as a sorus, which are so well seen in the 

 common Polypody. These sori, except in two small 

 genera, Polypodium and Gymnogramme, are protected, being 

 sheltered by an umbrella-like covering, known as the 

 indusium, which is either circular and button-like, or 

 kidney-shaped, or continuous with the reflexed margin of 

 the frond itself. 



In the genus Polystichum, our shield-ferns now under 

 notice, the indusium which covers the s2)oramjium-ii\\xB\.Qv is 

 orbicular, i.e. circular, and it is also peltate, i.e. target- 

 shaped or shield-like, in being attached by the centre of 

 its under surface to the surface of the fern itself by a 

 stalk. This shield-shaped indusitim is the cause of the 

 plants being called shield-ferns. 



Of the genus Polystichum there may be said to be three 

 British species, viz. Lonchitis, aculcatum, and anyvlare. 

 We are at present showing the last-named, angidare, 

 because of the fact that it is a plant of apparently increas- 

 ing rarity, " whether," as Mr. Arnold Lees remarks, from 

 being " a running-out species, or one that has decreased from 



TBANS. BOT. SOC. EDIN. VOL. XXII. X 



