GROUP I. THALLOPHYTA : FUNGI. 277 



at ion) (e.g. the Yeast-forms of various Fungi). It is effected in 

 some cases (e.g. in some Zygomycetes, Ascomycetds, and Basi- 

 diomycetes) by unicellular gcmmce of various .sizes (termed chlamy- 

 dospores when they are relatively large and thick -walled; and 

 are adapted for a period of quiescence ; oidium-cells, when they are 

 small and thin-walled and capable of immediate germination) 

 which are formed by the segmentation of a hypha by transverse 

 septa into short cells which become somewhat rounded and separate 

 from each other ; on germination, each may give rise to a mycelium. 

 In other cases (e.g. many Ascomycetes, such as the Sclerotiniese, 

 Pezizese, Claviceps, etc. ; some Basidiomycetes, such as Coprinus 

 stcrcorarius, species of Typhula and Agaricus), it is effected by 

 bodies termed sclcrotia ; each sclerotium consists of a compact 

 mass of hyphge, filled with reserve materials, covered by a cortex 

 of one or more layers of tissue, which are thick- walled and of a 

 dark colour. They become detached from the mycelium on which 

 they are formed, and are capable of retaining their vitality during 

 a long dormant period ; on germination they give rise to shoots 

 bearing reproductive organs. A form of sclerotium is found also in 

 the Myxomycetes. Here it consists of a plasmodium, or a part of 

 a plasmodium, which has surrounded itself with a membrane, and 

 remains for a longer or shorter time in a dormant condition : 

 the individual amoeboid cells may also surround themselves with 

 a membrane and remain dormant, in the form of microcysts. 



Reproduction is effected sexually or asexually. A sexual process 

 takes place in the Zygomycetes, in the Peronosporacese, and in 

 some Ascomycetes. 



The modes of the sexual process are the following : 



I. Isogamy : sexual cells, similar aplanogametes which are not 

 set free ; process, conjugation ; product, a zygospore ; Zygomy- 

 cetes. 



II. Heterogamy: 



a. Oogamy : sexual cells, oospheres and undifferentiated male 



cells ; process, fertilisation ; product, an o'ospore ; Peron'o- 

 sporacese. 



b. Carpogamy : no differentiated female cell ; female orga'ri 



fertilised by the undifferentiated contents of the male organ 

 or by differentiated male cells, spermatia : product, a fructifi- 

 cation termed an ascocarp : all the forms in which this 

 mode occurs belong to the Ascomycetes. 



