PTERIDOPHYTA. 711 



402 ^). The arrangement of the sori is not unlike those in Trichomanes (cf. fig. 

 400^), except that the cup is closed round the sorus. In each sorus occur only 

 macrosporangia or microsporangia; but in each group of three sori usually one is 

 different from the other two. Each sorus with its investment constitutes a sporo- 

 carp. The other genus, Azolla, resembles a floating, leafy Jungermannia {cf. fig. 

 396 ^ p. 698); it is closely set with tiny leaves, and numerous true roots hang down 

 into the water. The macrospores are provided with a floating apparatus and hooks ; 

 and the microspores which escape from their sporangia in packets have long barbed 

 appendages, which become attached to the hooks of the macrospores. Thus the 

 spermatozoids escape in the immediate neighbourhood of the female prothallia. 



There are 9 species of Salviniaceae. 



Fossil residues occur in the tertiary formations. 



Marsiliacew. — Containing the two genera, Pilidaria and Marsilia. Both grow 

 in marshy or inundated ground, and spread their rhizomes horizontally, attaching 

 them by means of roots. The leaves in Pilidaria (fig. 402 ^) are needle-like, and 

 each bears at the base a very short branch which develops into a sporocarp. In 

 Marsilia (fig. 402^) the leaves resemble those of Oxalis; near the base they give ofi' 

 a branch which may bear several bean-like sporocarps. The sporocarps in both 

 these plants do not — as in the Salviniaceae — consist of mere sori with an investment ; 

 but each is a leaf -segment in which a number of cavities develop (four in Pilularia, 

 many in Marsilia), cavities which ultimately are quite cut off from the exterior, 

 though they arise at first as pittings of the surface. In these cavities groups of 

 sporangia arise — both macro- and microsporangia in each chamber. The sporocarp 

 in this family is, therefore, in nature a leaf -lobe containing numbers of sporangial 

 cavities, and of much greater complexity than in the Salviniacese. The sporocarps 

 ultimately dehisce, the spores develop their prothallia, and fertilization takes place. 

 There are 32 species of Marsilia and 3 of Pilularia. P. Glohidifera alone is 

 British. Fossils are found in tertiary formations. 



Alliance XXIV. — Equisetales, Horsetails. 



Possess jointed stems and small leaves inserted in whorls. The sporangia are 

 produced on special leaves arranged in cones. All living examples are homosporous, 

 but palseozoic forms include heterosporous genera. 



Families: Equisetaceoe, Galamarioi. 



The Eqvbisetacem alone are represented by living plants, and include the solitarj^ 

 genus Equisetwm, with about 40 species. 



The habit of growth of the Equisetums is exceedingly characteristic. There is 

 a branching underground rhizome from which erect aerial shoots are produced each 

 year. From the nodes of the underground stems numerous fine roots arise (fig. 

 403^). The whole of the aerial shoot is green and assimilating, and the leaves are 

 represented by funnel-shaped sheaths bearing teeth inserted at the nodes. The inter- 

 nodes are ribbed and the whole structure harsh to the touch, and often l^rittle owing 



