THALLOPHYTES 61 



individuals (gametophytes) contain the half number (x). This means 

 that in passing fromsporophyte to gametophyte, 200 must be reduced to x. 

 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 



