46 INTUODIK riON TO I'l.ANT C I'O (; R A I' II Y [cHAP. 



actual conipoiunts of vegetation their role is usually minor, unless 

 it be very locally and temporarily when food supplies are plentifid. 

 However, in economic connections their importance is vast and 

 multifarious : the frequency with which they cause diseases, especi- 

 ally of plants, has already been referred to. Such diseases result in 

 many hundreds of millions of dollars' worth of damage to crops 

 yearly in North America alone. Also explained above is their 

 significance as scavengers, returning the products of organic break- 

 down to the air and soil in simple forms that can be absorbed and 

 used again by green plants. Many other saprophytic I*\mgi are 

 outstanding nuisances to Man in causing spoilage of food and 

 destruction of fabrics, timber, and so forth. Yet others are valuable 

 in positive ways, examples being the activities of Yeasts, which are 

 employed in the making of wines, beer, and bread, and the use 

 of Mushrooms and 'Toadstools as human food. There are also the 

 ' industrial ' Fungi that provide valuable sources of certain food 

 proteins and vitamins, and the ' medicinal ' ones that provide such 

 antibiotics as penicillin. Mnally, T'ungi play a significant role in 

 the nutrition of many higher plants- including forest trees, in or 

 upon whose roots tlu'N' li\e and form mycorrhizas.' 



The above nine groups, which we have treated as classes, are 

 considered by many authorities to be subdivisions (subphyla) or, 

 especially in some instance, full divisions (phyla). The same is true 

 of a few other, smaller groups which are of very little importance 

 phvtogeographically, or as components of vegetation, and which we 

 are aecordingh' ignoring. 



Liciii'Ni'S : These are the Lichens peculiar tlual organisms 

 produced by the intimate association of two plants, a T'ungus and 

 an Alga or a Schizophyte, and accordingly belonging to different 

 groups. They seem best treated as a separate class or subdivision 

 of the Thallophyta. Such a ' living together ' for mutual benefit 

 is termed a svmbiosis, and of this Lichens afford the great example, 

 though mycorrhizas (.svr aboxe) are another. In the formation of 

 Lichens the fungus usually forms a tough, often leathery, invest- 

 ment, the algal or schizopln te cells or filaments being interspersed 

 or grouped within, most tNpically forming a layer near the upper 

 surface. Lichens are often luxuriant ant! may li\e for centuries, the 

 symbiosis being evidenth' a nuitually beneticial relationship in that 



' l'\)r ai\ iip-li)-d;iU' accduiil ol ihis inlrimiiiiu siilijcil, .«<' Dr. J. L. 1 lailoy's 

 Tlie Biology 0/ Myconliizd (1 Annanl llill, I.oiulon, pp. .\i\ | 233, 1959). 



