GROUP I. THALLOPHYTA : FUNGI. 287 



thick- walled and large; the oidium-cells are smaller and thin-walled 

 (see p. 277). 



Mucor Mucedo may be obtained by keeping fresh horse-dung under a 

 bell-jar in a warm room: it may be further cultivated by sowing the 

 spores on slices of bread moistened and kept under a bell-jar. 



Section B. Oomycetes. 



This section of the Phycomycetes includes the following orders : 

 Order 1. Peronosporaceae: body branched; oogonia terminal 



or intercalary ; pollinodium functional. 



Order 2. Saprolegniacese : body branched ; oogonia generally 

 terminal, rarely intercalary ; pollinodium absent, 

 or, if present, functionless. 



Order 1. Peronosporaceae. The forms comprised in this order are 

 mostly parasitic, chiefly on Phanerogams, but some species of Pythium 

 inhabit the dead bodies of plants and animals. 



The asexual reproduction of the plant is effected, in most forms, by 

 sporangia developed at the ends of the branches of the simple sporophores 

 (Fig. 168 A): no such organs have, however, been 

 observed as yet in Pythium vexans or P. Artotrogus. 

 In some forms the sporangium gives rise to zoo- 

 spores either before or after, it has fallen off the 

 sporophore (Fig. 168 B, C) ; whilst in other forms 

 it falls off and germinates as if it were itself a 

 spore, growing out into a hypha, and so into a p IG- 167- _ 

 mycelium. phthora omnivora. An 



The oogonium is spherical, and remains closed oogonium (Og), contain- 



(Fig. 167). The protoplasmic contents undergo *** an spore ( 'f ' a 

 \" . . . ' . . a pollinodium which has 



differentiation into a single oosphere which is fert iii 8e d the oosphere. 

 surrounded by the remainder of the protoplasm, (x400.) 

 the peripi-asm. 



The pollinodium is developed terminally, either on a hypha springing 

 from beneath the 'oogonium, or on an adjacent hypha, and is club-shaped- 

 Its protoplasmic contents undergo differentiation into a male cell (aplano- 

 gamete) and into periplasm. 



At the time of fertilisation, the pollinodium is closely applied to the 

 oogonium and sends out a delicate tube which penetrates through the 

 wall of the oogonium and reaches the oosphere. The tube then opens, and 

 the male cell passes out of the pollinodium into the oosphere and fertilises 

 it. The oosphere then surrounds itself with a proper wall and becomes 

 the oospore. In some genera (Peronospora, Cystopus) an external coat, 

 the episporium or perinium, is formed round the oospore from the peri- 

 plasm. 



The germination of the oospore takes place in different ways in different 

 species. In Phytopltthora omnieora and Pythium proliferum it gives rise 

 to a small mycelium (promycelium) which produces a few spores, from 



