476 HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



we have the young fern-plant with its fronds (c/. fig. 346 ^). The fern-plant bears 

 no sexual organs, and must be regarded as the asexual generation (or sporophyte). 

 Its first fronds provide the necessary food-materials for the production of new 

 fronds, which arise in increasing numbers from the stem-apex; as a rule the stem 

 remains short, or it may be elongated horizontally as a rhizome, or, in the Tree 

 Ferns (c/. fig. 347), it develops into an erect caudex bearing a tuft of green fronds at 

 its apex. In addition to their purely assimilating function, the fronds are concerned 

 in the propagation of the plant, and produce quantities of spore-cases (or sporangia) 

 containing spores. These sporangia arise in clusters, known as sori, and are usually 

 situated on the under sides of the fronds (see figs. 346 ^ and 346 ^, and fig. 189, p. 11). 

 In the majority of Ferns these two functions — assimilation and spore-production — 

 are performed by one and the same frond, and there is no especial difierence in 

 structure between the assimilating and spore-producing portions. But in the 

 so-called " Flowering Fern ", or Royal Fern (Osmunda regalis), these two portions 

 of the frond stand out in marked contrast; the topmost pinnules of the frond are 

 entirely covered with sporangia, and light brown in colour, whilst the lower portions 

 are bright green, and quite destitute of sporangia. In the Hard Fern (Blechnum 

 Spicant) and Parsley Fern {AUosorus crispus) there is a distinction between the 

 sterile and fertile fronds, the pinnules of fronds which bear sporangia being much 

 narrower than those of purely assimilating fronds. In Rhipidopteris peltata, again, 

 the fertile fronds are disc-like, whilst the assimilating fronds are branched and 

 filamentous (see fig. 348); in Platy cerium alcicorne the fertile fronds are branched 

 like a reindeer horn, whilst the sterile ones form great green discs in close contact 

 with the bark of the tree on which it grows, and remind one of huge prothallia 

 (see fig. 349). As soon as the spores are mature they are discharged from the 

 sporangia and scattered by the wind. Falling on moist earth, on the bark of a 

 tree, or in a rocky cleft, they germinate, producing prothallia, upon which the 

 sexual organs are borne (c/. fig. 346 ^). Thus in the Fern, two stages are well shown 

 in the life-cycle, (1) the prothallium, the sexual generation or oophyte, and (2) the 

 fern-plant, the asexual generation (or sporophyte), which bears spores, these in turn 

 give rise to the first generation again. 



In the Horsetails (Equisetacese), which have been figured and referred to at 

 p. 14, a similar alternation of generations occurs. The fern itself is the asexual 

 generation, and bears cones of sporangium-producing scales. From the contained 

 spores prothallia are formed. In several species of Horsetail (e.g. Equisetiim 

 sylvaticum, fig. 190'^, p. 14) one and the same shoot bears the organs of assimilation 

 and spore-production; whilst in other species (e.g. Uquisetum arvense) these functions 

 are relegated to distinct shoots; i.e. shoots formed in spring, which terminate in 

 cones (fig. 190^, p. 14), and others formed later, which bear numerous green assimi- 

 lating branches, but no cones (fig. 190^, p. 14). 



In the group of the Lycopodinse very interesting conditions prevail. In the 

 so-called Club Mosses (Lycopodiaceae) the plant is much branched, and in a great 

 many species of Lycopodium (e.g. Lycopodium annotinum, fig. 378) the shoots end 



