430 BOTAXY OF THE LIVING PLANT 



upon them. Now, some of the Lichen-Fungi have been cultivated in 

 nutrient media when they produce a small thallus without gonidia. 

 And again, the algal cells have been isolated and have been found to 

 continue their normal life as free- growing Algae. Indeed they have 

 been found to agree in detail with certain species of the lower Algae. 

 Finally, synthetic experiments have been successfully carried out in 

 building up a Lichen from its two constituents when grown together. 

 The proof that the Lichen consists of a coalition of two organisms 

 living in symbiotic relation seems thus complete. That the symbiosis 

 is a mutual advantage is clear from the healthy growth. It may be 

 held that the alga contributes fresh organic substance by photo- 

 synthesis, while the fungus supplies water and soluble salts, which it is 

 specially able to extract and convey. 



