CHAPTER VI 

 THE FUNGI 



THE Subkingdom Fungi includes a very large number of plants, 

 nearly forty thousand species having already been described. These 

 differ primarily from the Algae in being destitute of chlorophyll, but 

 there are also very marked structural differences. Owing to the 

 absence of chlorophyll, they are incapable of assimilating C0 2 . This 

 absence of chlorophyll is not, probably, a primitive condition, and 

 they are presumably derived from algal forms with chlorophyll. 

 Doubtless many of the peculiarities of the Fungi are secondary ones 

 connected with their peculiar habits, necessitated by their dependence 

 upon organic food. A small number of Fungi, the Phycomycetes, 

 show more or less obvious evidences of their algal ancestry, but much 

 the larger number have become so modified as to leave little or no 

 traces of structural resemblances to other plants. Where the Fungus 

 lives upon dead matter, it is known as a saprophyte ; where it attacks 

 living plants or animals, a parasite. 



A few Fungi are aquatic, but most of them live either within the 

 bodies of their hosts or within the nutrient medium upon which they 

 feed. 



Parasitism. Many Fungi, such as the Rusts, Smuts, and many 

 Mildews, are absolutely dependent upon living organisms, so-called 

 " obligate parasites." Less frequently a Fungus which is ordinarily 

 a saprophyte may assume parasitic habits ; i.e. it becomes a " facul- 

 tative" parasite. While some species of Fungi are dependent upon 

 a specific host, more commonly they may grow upon several some- 

 times many different hosts. Some forms, like certain species of 

 Rusts, in the course of their development live upon two hosts, often 

 quite unrelated. Thus the Cedar-rust (Gymnosporangium) passes 

 part of its life upon the Red Cedar, and part upon the Crabapple or 

 Hawthorn. This change of host, or " Hetercecisin," is curiously 

 like the behavior of certain animal parasites, like the Tapeworm and 

 Trichina, which live successively in the bodies of different hosts. 



Symbiosis. A special form of parasitism, called Symbiosis, is 

 exhibited by a number of Fungi. The most familiar of these are 

 the Lichens, where a Fungus is intimately associated with an Alga, 

 upon which it is parasitic to a greater or less extent, but to which it 

 affords shelter, and probably certain food-elements, so that the asso- 

 ciation is to some extent mutually advantageous. 



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