SECT, i CRYPTOGAMS 331 



of cell division within certain cells, which are known as sporangia ; 

 in other cases they arise by modification and separation of cells of 

 the thallus or by a process of cell-budding. When the spores possess 

 cilia and are able to move actively in the water, they are known as 

 swarm-spores (zoospores) ; when they do not bear cilia they are 

 termed aplanospores. In the latter case the spores if distributed 

 by water may be naked, or they may be provided with a cell-wall 

 and suited for distribution in the air. 



Sexual reproduction is also of wide-spread occurrence. It 

 consists, in the simplest cases, in the production of a single cell, 

 the ZYGOSPORE or ZYGOTE, by the union or conjugation of two 

 similarly formed sexual cells or gametes. The organs in which 

 the gametes are formed are termed gametangia ; planogametes 

 are provided with cilia while aplanogametes are non-ciliated. In 

 many of the more highly developed forms, however, the gametes are 

 differentiated as small, usually ciliated, male cells or SPERMATOZOIDS, 

 and as larger non-ciliated female cells, the egg-cells or OOSPHERES. 

 The spermatozoids are formed in antheridia, the oospheres in oogonia. 

 As a result of the fusion of an egg-cell and a spermatozoid, an 

 OOSPORE is produced. The first form of sexual reproduction or 

 fertilisation is termed ISOGAMOUS, the second OOGAMOUS ; but these 

 are connected by intermediate forms. It must be assumed that the 

 sexual cells have been derived in the phylogeny of plants from 

 asexual spores, and that asexual multiplication has taken origin from 

 simple cell division. The gametangia, oogonia, antheridia, and 

 sporangia of the Thallophyta are homologous structures. The sexual 

 reproduction has originated independently in several distinct groups. 



While the reproduction of some Thallophyta is exclusively asexual, and of others 

 exclusively sexual, in many others both forms of reproduction occur. In the latter 

 case this may occur on the one plant, or separate successive generations may be 

 distinguishable. Generally speaking, there is, however, no regular succession of 

 asexual and sexual generations in Thallophytes, the mode of reproduction being to 

 a great extent under the influence of external conditions ( 1 ). Only in some Brown 

 Sea- weeds, in the Red Sea-weeds, and some Fungi is there an alternation of a 

 sexual generation (gametophyte) with an asexual (sporophyte), such as is found 

 in all Bryophytes and Pteridophytes. 



CLASS I 

 Bacteria ( 2 ) 



Bacteria are unicellular or filamentous organisms of very simple 

 construction. Chlorophyll is wanting in them, and their mode of 

 life is consequently a parasitic or saprophytic one. A large number 

 of species exist distributed over the whole earth, in water, in the 



