THE FUNGI 85 



large numbers as the fungus grows older, but these are 

 borne upon branches situated deep down within the 

 host. Fertilization is effected by the formation of a 

 fertilizing tube as in the water-moulds (Fig. 21, I). 

 After the ripe oospore is set free by the decay of the 

 tissues of the host, it germinates by forming zoospores, 

 much as do the non-sexual spores. 



There are now known a number of algse which are 

 more or less parasitic, and which in their manner of life 

 suggest these parasitic Phycomycetes which may very 

 well have originated from similar algse. Such a parasitic 

 alga is Phyllosiphon, which is a genuine parasite within 

 the tissues of the leaves of a species of Arisarum, where 

 it causes considerable damage. This plant is a most 

 interesting example of an alga on the way to become 

 a fungus. It still possesses some chlorophyll, but that 

 it is a true parasite is at once shown by the injury 

 which it inflicts upon the host. A probably analogous 

 case among the flowering plants which possess chloro- 

 phyll is seen in such semi-parasites as the mistletoe and 

 Gerardia. 



Of the Phycomycetes which show less evident rela- 

 tionship to the algse, the most familiar are the black- 

 moulds. In these the structure of the plant is much 

 like those already described, i.e. a branching, but un- 

 divided, tubular thallus. The reproductive parts are, 

 however, quite different, none of the reproductive cells 

 ever showing motion. The sexual cells, or gametes, 

 are usually alike, and fertilization is effected by the 

 fusion of two similar cells (Fig. 22, D), somewhat as in 

 some of the desmids and pond-scums among the algse. 

 It has been suggested that possibly the black-moulds 



