THALLOPHYTES 61 
individuals (gametophytes) contain the half number (x). This means 
that in passing fromsporophyte to gametophyte, 2x must be reduced tox. 
In Polysiphonia, the sexual individuals (gametophytes) show twenty 
chromosomes in their nuclear divisions, and of course this number 
characterizes the male and female nuclei which they produce. At 
fertilization the fusion nucleus receives forty chromosomes, and this 
number persists through the cystocarp, the carpospores, and the tetra- 
sporic plants. This indicates that the tetrasporic plants are true sporo- 
phytes, and it is in the sporangium, in the nuclear divisions concerned 
in the formation of the tetraspores, that the reduction in the number of 
chromosomes occurs. These nuclear divisions are called reduction 
divisions, and they represent one of the two important epochs in the life 
history, the other being the act of fertilization. On account of the re- 
duction division, each tetraspore contains the half number of chromo- 
somes, and this number is continued through the sexual plant which it 
produces. In Polysiphonia, therefore, the male and female plants are 
gametophytes, and the tetrasporic plant is a sporophyte, the cystocarp 
also being sporophytic in the series of nuclei that extends from fusion 
nucleus to carpospore. 
4. FUNGI 
General character. This enormous assemblage of thallophytes is 
characterized by the absence of chlorophyll, resulting in a lack of power 
to manufacture carbohydrate food. As a consequence, they are either 
parasites, dependent upon living plants or animals as hosts ; or sapro- 
phytes, dependent upon organic debris or products from plants or 
animals. These are not terms of classification, for some fungi are able 
to live either as parasites or as saprophytes, and such are called faculta- 
tive forms; while those restricted to either the parasitic or the sapro- 
phytic habit are obligate forms. The possible range of parasitism is quite 
different in different forms, some parasites attacking miscellaneous hosts, 
others being restricted to closely related hosts, others to a single kind 
of host, and still others attacking only certain organs (see p. 381). 
The vegetative body of a fungus is the mycelium, composed of inter- 
woven filaments called hyphae. The mycelium may he very open and 
delicate, or it may be feltlike, or even form a compact body (as in 
lichens). The mycelium establishes absorbing connections with its 
food supply (the substratum), and when these connections are definite 
and more or less specialized, they are called haustoria (suckers). In the 
