LICHENS 133 



dioxide, water and salts, manufactures organic food for the 

 benefit of both organisms ; it also shares with the fungus in the 

 work of producing the brood bodies or buds (soredes). Both the 

 fungus and the algae which make up a fungus can, under suitable 

 conditions, be induced to grow independently, though in the lichen 

 itself they are very dependent upon each other. The isolated alga 

 cells grow and multiply when supplied with water, a few simple in- 

 organic salts, and air (containing carbon dioxide) . The lichen spores 

 will germinate in a culture solution containing organic substances 

 (sugar, etc.), and produce a small thallus which of course contains 

 no alga cells. In the lichen thallus the fungus has discarded the 

 usual fungus method of nutrition, which is either saprophytic 

 (i.e. using dead organic matter) or parasitic (i.e. using the living 

 substance of another organism), so that it can flourish on bare 

 rocks. An ordinary fungus, e.g. a mushroom, has its vegetative 

 part (mycelium or " spawn ") buried in the substratum, and grow- 

 ing away from the light and towards moisture, since it cannot 

 utilise sunlight for food making, and at the same time is pro- 

 tected from cold and drought ; only the spore fruits grow into 

 the air, that the spores may be dispersed. The behaviour of 

 a lichen is very different, as may easily be observed by noticing 

 the habitats in which the plants grow. 



The reproduction of a lichen is somewhat complicated, for 

 we have to consider the processes of multiplication carried on 

 by (i) the alga cells, (2) the lichen fungus, (3) the lichen as a 

 whole, (i) The alga cells increase in number by simply growing 

 and dividing as if they were living independently, but do not 

 bring about the reproduction of the lichen as a whole, which 

 is effected by the second and third methods. (2) In spore 

 production the fungus alone is concerned ; the spores are usually 

 formed in open cup-like fruits, though sometimes the fruits are 

 flask-shaped with a small opening, or long and then sometimes 

 branched (Gr aphis). The spores are carried away by the wind, 

 and on reaching a suitable place begin to germinate. Unless, 

 however, the threads put out by the spore meet with the proper 

 alga cells they cannot develop further. If the right algae are 

 present the fungus threads surround them, and a lichen thallus 

 is gradually woven. By sowing lichen spores on glass plates 



