704 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



numerous that it is hardly possible to mention even the most notable here. Fontin- 

 alis is of interest in being aquatic; Splachnum {cf. fig. 399) in having a very large 

 apophysis and being saprophytic on animal excreta (cf. vol. i. p. 118). Buxhaumia 

 aphylla is an exceedingly simple form and vegetates in the protoncma-stage. Leafy 

 shoots are only formed in connection with sexual reproduction, and even then they 

 are very rudimentary. This plant has been thought to be a primitive type of Moss. 

 Fossil Mosses are met with in Tertiary and more recent deposits. 



Class II.— PTERIDOPHYTA, Vascular Cryptogams. 



As in the Bryophyta, so here, a well-marked alternation of generations is ex- 

 hibited in the life-history. Whilst in the Liverworts and Mosses the oophyte is the 

 dominant stage ("the plant") here the sporophyte constitutes "the plant". The 

 oophyte is a mere prothallium of simple nature, the sporophyte is a complex struc- 

 ture with root, stem, and leaves, and a well-marked vascular system. It becomes 

 free from the prothallium at an early stage in development. The Pteridophyta 

 contain the following alliances: Filices, Hydropterides, Equisetales, Lycopodiales. 



The plant or sporophyte generation attains to a wide diversity of form in the 

 Pteridophytes; thus, amongst the Ferns the stem is often short and bears a rosette 

 of fronds, or is elongated and rhizome-like with leaves at intervals; in the Equiset- 

 ales it is erect and jointed, and the leaves are reduced to toothed sheaths; and in 

 many Lycopodiales the stem is procumbent, much -branched, and covered with 

 simple scale-like leaves over the entire surface. Upon the leaves are borne the 

 sporangia which contain the spores. The sporangia may be either scattered over 

 ordinary leaves or on special leaves collected into cones. There is one feature con- 

 nected with the spores that must be described here. Though in the Ferns and in 

 many other Pteridophytes all the spores are of one kind and each gives rise to a 

 prothallium bearing both archegonia and antheridia, there are Pteridophytes in the 

 alliances Hydropterides and Lycopodiales in which two sorts of spores are produced. 

 The latter are known as heterosporous, the former as homosporous. Where the 

 plants are heterosporous the spores are of two sizes, and the larger ones (macrospores) 

 are contained in fewer numbers in the sporangia than are the smaller ones (micro- 

 spores). On germination the macrospore gives rise to a female prothallium only, 

 the microspore to a male prothallium; i.e. growths which bear respectively arche- 

 gonia and antheridia. The male prothallium is a very simple structure, and its part 

 is played so soon as it has liberated its spermatozoids. The female prothallium 

 having to nourish the young sporophyte for a while, until such time as it can live 

 independently, is larger, and is usually well-provided with food-material. 



Contrasting the Pteridophytes and Bryophytes, the Fern-plant corresponds to 

 the sporogonium of the latter and the prothallium to the Moss-plant or Liverwort 

 thallus. In the former the sporophyte, in the latter the oophyte generation is the 

 more complex. But that a Fern-plant has been elaborated out of a Moss-sporo- 

 gonium, or that the Fern-prothallium is a reduced or degraded Moss-plant, is exceed- 



