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 odspore is set free by the decay of the 
tissues of the host, it germinates by forming zodspores, 
much as do the non-sexual spores. 
There are now known a number of alge 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 alge. 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 alge, the most familiar are the black- 
moulds. In these the structure of the plant is much 
like those already described, 7.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 alge. 
lt has been suggested that possibly the black-moulds 
