448 THE BRITISH NATURE BOOK 



they are rolled up upon themselves, and each leaflet (pinna) is also rolled up 

 upon itself. This is called circinate, and is characteristic of all the British 

 ferns except the Adder's Tongue and Moonwort. 



But the characteristic which is most noticeable and interesting is that 

 connected with the organs of fructification. 



If you examine the under side of a mature fern, you will notice a number 

 of minute brown patches, grouped either along the sides of the veins (dorsal), 

 or along the edge of the frond (marginal}. 



These patches are called sori, and are in reality clusters of minute spores. 

 In a very few cases, however, such as the Osmunda regalis and the Adder's 

 Tongue, a whole frond becomes the receptacle and, in popular parlance, becomes 

 & flower. 



These spore-cases should be examined with your pocket lens or microscope. 

 They are surrounded by a ring, fixed at one end to the frond. When the spores 

 are ripe, the case bursts across, and the spores are scattered. In the case of 

 three species, Osmunda, Adder's Tongue, and Moonwort, the spore-cases are 

 not ringed, but consist of two valves. 



Notice that in many cases the sori are covered at first by a thin, skin- 

 like membrane, or scale, known as the indusium, which is cast off as the 

 spores become ripe. The presence or absence of the indusium is a means of 

 identification. 



The after history of the spore is very interesting. It does not immediately 

 become a fern, but expands on the earth or rock to which it is attached, in the 

 form of a green, minute, leaf-like body, known as the prothallus, and in time 

 two sets of organs grow out, from which in due course the young fern is produced. 



Keeping these points in mind, it is not a difficult matter to identify the 

 common ferns of this country. Notice first of all the method of fructification. 



(a) If it is in a terminal spike or panicle, it will be either Osmunda (the 

 Royal or Flowering Fern), Ophioglossum (Adder's Tongue), or Botrychium 

 (Moonwort). 



(b) If it is in a tiny cup at the edge of the frond, the species is Hymenophyllum 

 or Trichomanes (the latter to be found only in Ireland). 



(c) If it is on the back or under side of some or all the fronds, then it belongs 

 to one of the remaining species, and a further means of identification must be 

 used : you must now notice the shape of the frond, the shape and position of 

 the sori, and the presence or absence of the indusium. 



The following notes give some of the main distinctions of the genera of 

 British ferns : 



1. POLYPODIACE.E:. Sori round, without indusium. 



(a) If the sori are dorsal, circular, and exposed Polypodium. 



(b) If the sori are dorsal, roundish, and running together beneath the mar- 

 gins of the frond Allosorus. 



2. GYMNOGRAMME.E. Sori linear, without an indusium. 

 Sori are dorsal, linear, forked, and naked Gymnogramma. 



