15.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 69 



There are two principal modes of origin of spores, and all plants 

 produce spores in one or other of these modes. In the one, the 

 spores are formed from the protoplasm of any part, or of some 

 special part, of an organism ; in the other, they are formed by the 

 fusion of two masses of protoplasm derived either from two dis- 

 tinct organisms, or from distinct parts of the same organism. In 

 the former case they are said to be formed asexually, ; in the latter, 

 they are formed sexually, the fusion of the two masses of proto- 

 plasm being a sexual process (p. 3) ; the organs concerned are 

 distinguished respectively as asexual and sexual, and are in all 

 cases confined to the shoot. 



In the Muscinese and all the higher plants, the asexual formation 

 of spores is absolutely restricted to the sporophyte ; and the 

 sexual formation of spores is absolutely restricted to the gameto- 

 phyte. But in lower plants (Algae, Fungi), the sexuality of the 

 gametophyte is frequently so far incomplete that it still retains 

 the power of producing spores (distinguished as gonidia) in a 

 purely asexual manner (see p. 3). 



The spore (or gonidium) is generally a single cell, consisting of 

 a nucleated mass of protoplasm containing various nutritive sub- 

 stances (oil, starch, etc.) : but in some cases (e.g. zoogonidium of 

 Vaucheria) where it is multinucleate, it is a coenocyte (see Part 

 II., Introduction). 



The spore frequently has a cell-wall, which is commonly thick, 

 and in some cases consists of two layers, an outer, the exine (or 

 exospore), which is cuticularised, and an inner, the (inline or en- 

 dospore), which is delicate and consists of cellulose. 



In some cases the spore has no cell-wall. It may then be 

 capable or incapable of spontaneous movement. When motile, it 

 either swims by means of one, two, four, or many cilia, or it 

 creeps in an amoeboid manner by means of pseudopodia. Motile 

 spores are termed zoospores (or zoogonidia). Ciliated zoospores 

 are common among the Algae (e.g. Hgematococcus, Vaucheria, 

 Ulothrix, Botrydium, Ectocarpus), and they occur in some Fungi 

 (Saprolegnieee, Peronosporeae). Amoeboid zoospores occur in the 

 Myxomycetes among the Fungi ; in Chromophyton, and in th^ 

 Floridean genera Helminthora, Bangia, and Porphyra (carpo- 

 spores), among the Algae. Non-motile naked spores occur only 

 in the Florideae ; both the asexually-produced (tetragonidia) and 

 the sexually-produced (carpospores) spores belong to this cate- 

 gory, with the exceptions mentioned above. 



