68 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. Lxii. 



such circumstances is by getting into co-operation with a 

 plant that can make organic matter. Algre are the most 

 convenient plants of that kind. They are provided with 

 chlorophyll, and in their chlorophyll cells proceeds the 

 wonderful process by which the carbon derived from the 

 carbonic acid of the air is assimilated so as to form sugar, 

 starch, and cellulose, and other organic matters. The small 

 green-coloured cells enfolded in the fungus substance of the 

 lichen are unicellular algte, which have been caught by it 

 in some way. They have no roots, and are unprovided 

 with the means of obtaining their mineral requirements 

 from the soil. Thus we see in the lichen two distinct 

 vegetable organisms closely associated for their mutual 

 benefit — a fungus, whose mycelium, acting the part of 

 roots, can find on the barest stone the mineral constituents 

 necessary for plant growth, and an alga possessed of the 

 power of obtaining from the air the carbon required to 

 build up the organic tissues of the plant. 



That the lichen is really a composite plant of that kind 

 has been proved in a very interesting way. Eees and 

 Stahl chose fungi of various kinds, and also algie of various 

 kinds, and by bringing them together brought about the 

 conditions of lichens quite similar to known species, an'd 

 by bringing together a species of lichen and a species of 

 alga that had not been seen associated together before, they 

 were able to manufacture what would have been called a 

 new species of lichen. In course of time the fungus, 

 having grown and nourished, and developed its organs of 

 reproduction and produced a large number of fertilised 

 spores, which are its seed, disintegrates and sets free the 

 unicellular algie, which on their part have increased in 

 number, and the two kinds of organisms are now ready to 

 be blown away by the wind or carried away by water, and 

 enabled to propagate a new generation of fungi and a new 

 generation of algic capable of leading single lives in suitable 

 circumstances, or of getting into symbiotic relationship of a 

 similar, or it may be of a quite different, kind from that 

 under whicli they had been nurtured. 



The second instance of plant symbiosis to which I 

 would shortly refer, is one which is not only interesting, 

 but of far-reaching importance. 



