372 ORGANOGRAPHY. 



When these spores germinate, a little pouch-like process pro- 

 trudes from their surface by an elongation of their membrane ; 

 this ultimately forms a green, lobed, flattened expansion, re- 

 sembling in all its essential characters the pro-thallus of a Fern. 

 Like Ferns also, this pro-thallus becomes furnished with anihe- 

 ridia containing ciliated spermatozoids, and archegonia. From 

 the embryo or germ-cell of the archegonium also, as in Ferns, a 

 new plant is ultimately produced resembling in every respect 

 that of the parent plant from which the spores were obtained. 

 As is the case in Ferns therefore, we have in the Equisetaceae 

 also, an instance of alternation of generations. 



3. Marsileace^ or Peppervtorts. — In the plants of this 

 order the fi-uctification is placed at the base of the leaf-stalks. 

 It consists usually of a two-valved stalked involucre or sporo- 

 carp {fig. 790), which is generally many-celled, or sometimes 

 one-celled, and appears to be a modified leaf. The contents 

 of the sporocarps, and the mode in which they are arranged, 

 differ somewhat in the different genera of this order, and 

 hence it will be necessary for us to allude to them separately. 



In Marsilea, the fructification consists of a stalked, two- 

 valved, hardened sporocarp {fig. 790, s). The valves are held 

 together by a mucilaginous ring, Avhich is at first connected 

 with the stalk of the sporocarp, but when the latter organ 

 bursts, the ring becomes detached from the stalk at one end, 

 straightens, and appears as a long mucilaginous cord protruding 

 from the sporocarp {fig. 790, />), and bearing on its sides some- 

 what oblong spikes of fructification {fig. 790,/). These spikes 

 are at first enveloped in a membrane, and are composed of two 

 distinct organs, called antheridia, and pistillidia sporangia or 

 ovules ; these are attached to a sort of placenta, the antheridia 

 being on one side, and the sporangia on the other. 



Each sporangium contains but one spore. It consists of a 

 central nucleus, surrounded by a cellular coating except at 

 its apex, Avhere there is a little cavity {fig. 792). Accord- 

 ing to Hofmeister, as quoted l)y Berkley, " this cavity is 

 gradually filled up with cellular tissue, constituting a conical 

 pro-thallus confluent with the nucleus. A single archegonium 

 is formed in the centre, the orifice of which corresponds with 

 the apex of the pro-thallus." In this an embryo is ultimately 

 formed, which, when it germinates, gives off" a frond in one 

 direction and a root opposite to it. 



The antheridia contain a number of small cells {fig. 791), 

 which ultimately develop long spiral spermatozoids. These 

 small cells are sometimes called pollen-spores or small spores, 

 while the large germinating spore is called the ovulary-spore or 

 large spore. 



In Filularia, the fructification consists of stalked, pill- shaped, 

 hairy sporocarps, situated at the base of the leaves. The in- 



