SEA WEEDS 345 



they exhibit. The first generation is a sexual one produced from 

 the spores, and consists either of a mass of delicate threads from 

 which a plant with a leafy axis is developed by a process of budding, 

 or of a little green frond (the thallus). These bear the male and 

 female elements, called respectively the antheridia and the 

 arcliegonia ; and when the central cells of the latter are fertilised 

 by the former, they give rise to a case, with or without a stalk, 

 containing a number of spores. When the case is ripe, it opens 

 horizontally by means of a lid, thus liberating the spores. 



Following these in the ascending scale are the Vascular 

 Cryptogams, in which some of the cells become modified into true 

 vessels. Here, too, the plants exhibit a distinct alternation of 

 generations, the spore first giving rise to a small, leafless body, 

 the prothallium, which bears the sexual organs ; and then the 

 female elements, after fertilisation, produce the spore-bearing 

 plant. 



This group contains quite a variety of beautiful and interesting 

 plants, including the Ferns (Filicales), Horsetails (E quisetales) , 

 Club-mosses (Lycopodiales), Water Ferns (Rhizocarpece),a,ndSela- 

 ginellales. 



Ferns usually produce their little green prothallia above ground, 

 and the perfect plant generally has a creeping rhizome or under- 

 ground stem. Some, however, have strong, erect, woody stems, 

 such as we see in the tree ferns of tropical and sub-tropical countries. 

 The horsetails and the club-mosses are also produced from prothallia 

 that are formed above ground. The perfect plants of the former 

 have branching underground stems which give off numerous roots, 

 and send up annually green, jointed, aerial stems that bear whorls 

 of fine leaves, each whorl forming a toothed, ring-like sheath. The 

 fertile shoots terminate in cones, on the modified leaves of which 

 the sporangia are produced. The stems of the club -mosses are 

 clothed with small overlapping leaves, in the axes of which the 

 sporangia are produced ; and the spores, which are formed in 

 abundance, constitute the lycopodium powder with which druggists 

 often coat their pills. 



' Water ferns either float on the surface of water or creep along 

 the bottom, and produce their fruit either at the bases of the leaves 

 or between the fibres of submerged leaves. The Selaginellas are 

 characterised by a procumbent stem that branches in one plane 

 only, producing small, sessile leaves, with a single central vein. 

 A number of roots grow downward from the under side of the stem, 



