SECT. 1 CRYPTOGAMS 333 



in other cases they aiise 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 ai-e 

 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 (isogamy). The organs in 

 which the gametes are formed are termed gametangia ; planogametes 

 are provided with cilia while a})lanogametes 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 oos2:)heres in OOGONIA. 

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

 oospore is produced (oogajiy). It must be assumed that the 

 sexual cells have been derived in the phylogeny of i)lants 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 otliers 

 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 Thalloi)hytes, the mode of reproduction being to 

 a great extent under the influence of external conditions (^). 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 tlie two sexual cells the fusion nucleus obtains the double 

 uumlier of chromosomes ; it becomes diploid while the sexual cells always have 

 HAPLOID nuclei. A reduction division of the diploid nucleus to the hajiloid 

 must thus occur in the course of the ontogenetic development. In most cases this 

 happens on the germination of the zygospore or oospore. In some Brown Algae 

 with an alternation of generations and similarly in all archegoniate plants a dijiloid 

 sporophyte is first developed from the germinating oospore and the reduction 

 division takes place in the asexual sporangia which the sporophyte bears. From 

 the haploid spores of these sporangia, haploid garaetophytes in turn develop. 



