DIV. i THALLOPHYTA 369 



of the Green Algae ; they appear to be connected in important characters with 

 the Brown Algae. 



The Thallophytes are commonly multiplied and distributed by 

 asexually produced SPORES, the mode of development of which differs 

 in the several groups. In many cases the spores arise by a process 

 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 widespread 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 (iSOGAMY). 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, 

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

 The spermatozoids are formed in ANTHERIDIA, the oospheres in OOGONIA. 

 The zygote which results from the fertilisation of an oosphere by a 

 spermatozoid is known as an OOSPORE when it passes into a resting 

 condition ; it may, however, in certain groups commence its develop- 

 ment at once. It must be assumed that the sexual cells have been 

 derived in the phylogeny of plants from asexual spores. 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 ( 2 ). 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. 



In the union of the two sexual cells the fusion nucleus obtains the 'double 

 number of chromosomes ; it becomes DIPLOID while the sexual cells always have 

 HAPLOID nuclei. A REDUCTION DIVISION of the diploid nucleus to the haploid 

 must therefore occur in the course of the ontogenetic development and a distinc- 

 tion can thus be made between a haploid and a diploid phase in the life-history 

 of the plant. The reduction division in many groups of Thallophyta takes place 

 in the germinating zygote. It may, however, occur at different stages even in the 



2B 



