62 MORPHOLOGY 



case of a parasite, the substratum is either the surface or the internal 

 tissues of the host (internal or external parasites), and in such cases the 

 haustoria are very distinct structures (figs. 1079, 1080). 



Under appropriate conditions the mycelium also produces vertical 

 branches (sporophores), which in a variety of ways give rise to spores. 

 In the case of internal parasites, the sporophores reach the surface of the 

 host, the spores thus being formed in surroundings that favor dispersal. 

 Fungi are notable for the vast number of spores produced, and in most 

 cases their dispersal is aerial, so that mycelia are multiplied with great 

 rapidity and over wide areas. The sexual reproduction of fungi is 

 exceedingly varied : in some cases the sex organs are as evident as are 

 those of algae ; in other cases the sexual act is so obscure as to raise the 

 question whether in some life histories it has not been eliminated entirely. 



Usually three great groups of fungi are recognized: (i) Phycomycetes 

 (algal fungi), (2) Ascomycetes (sac fungi), and (3) Basidiomycetes (basid- 

 ial fungi). The Phycomycetes differ so much from the other two 

 groups that the latter are often spoken of together as the Eumycetes, 

 or true fungi, and they contain the large majority of fungi. 



(i) PHYCOMYCETES 



General character. This comparatively small group of fungi re- 

 sembles the green algae in many features, a fact which has suggested the 

 name. It is not hard to imagine that the Phycomycetes are green algae 

 which have lost their chlorophyll and have developed the dependent 

 habit. Such a claim cannot be made for the Eumycetes, which have so 

 little resemblance to the algae that any connection with them is too ob- 

 scure to consider. The mycelium of Phycomycetes is composed of 

 coenocytic hyphae, suggesting a connection with Siphonales ; and this 

 connection with green algae is further emphasized by the sex organs, 

 which are equally prominent and of the same structure. In fact, the 

 two groups of Phycomycetes are distinguished by their sexual apparatus : 

 (a) Oomycetes, which are heterogamous, and (b) Zygomycetes, which 

 are isogamous. 



(a) Oomycetes 



General character. These heterogamous forms are regarded as more 

 primitive than the isogamous Zygomycetes because they are more closely 

 related to the algae. They are mostly aquatic and produce zoospores, 

 in these features differing from the Zygomycetes, in which the aerial 



