246 STRUCTURAL BOTANY 



Soon the filaments of the Fungus, well-fed at the 

 expense of the Alga, are strong enough to build up a 

 thallus. In the middle of Fig. 100 the hyphee are seen 

 uniting to form a network, which represents the beginning 

 of the cortical layer. 



Observations such as these have removed all doubt as 

 to the compound nature of the organisms called Lichens. 

 What, then, is the real relation between the Alga and the 

 Fungus of which the Lichen is built up ? It might be 

 supposed that the case is one simply of parasitism, the 

 Alga playing the part of a mere victim to the devouring 

 Fungus, just as a Cress-seedling is preyed upon by 

 Pythium or a Hop-plant by Sphcerotheca. This does not, 

 however, seem to be the real condition of affairs. The 

 Alga is not, on the whole, injured when the Fungus 

 annexes it. A few of the algal cells may be exhausted 

 and die, but the great majority live and go on multiplying 

 within the Lichen, quite as happily as if living at liberty 

 in the open air. It seems that there are advantages on 

 both sides ; the Alga, by the aid of its chlorophyll-bodies, 

 undertakes the whole duty of the assimilation of carbon, 

 thus providing the Fungus with the organic food which 

 it is unable to manufacture for itself. In many Lichens 

 there are definite pores in the upper cortex, allowing of 

 gaseous interchange between the atmosphere and the 

 gonidial layer. On the other hand, the rhizoids of the 

 fungal partner supply water and mineral food, probably 

 in a more effectual way than the Alga could obtain 

 them for itself. At the same time the tissues of the 

 Fungus shelter the Alga and protect it from the weather, 

 and especially from the effects of drought. It is probable 

 that many unicellular Algse, when enclosed in the thallus 

 of a Lichen, are able to exist, perhaps for centuries, in 



