102 PHYSIOLOGY OF NUTRITION 



even the inner tissue of the root not killed by the hot water. 

 Here and there on the mycelium we observe soft white balls, 

 which gradually increase in size, and ultimately surround them- 

 selves with a dark-coloured rind. These are sclerotia, from which 

 new sporophores may be obtained, though not, it is true, till after 

 a long period of dormancy. We also take a small quantity of the 

 Peziza mycelium, vigorously vegetating on a piece of carrot, and 

 introduce it into a small hole made in a vegetable marrow lying in 

 a covered glass. The mycelium is able to penetrate into the living 

 tissue ; it vegetates very luxuriantly and forms sclerotia, the 

 vegetable marrow becoming completely destroyed. 1 



1 Much information about parasitic fungi will be found in Frank, Krank- 

 heiten der Pflanzen, Breslau, 1880. 



40. Lichens. 



Lichens are well-known organisms which owe their existence to 

 the association (symbiosis) of fungi and algae. The relation in 

 general is that the algae in virtue of their chlorophyll produce 

 from inorganic material, by assimilation, the organic material 

 necessary both for their own existence and that of the fungi, while 

 the fungi afford the algae protection, more especially against de- 

 siccation. The lichen thallus maybe homoiomerous or heteromer- 

 ous in construction. To acquaint ourselves with the structure of 

 these remarkable organisms, we will take for examination a repre- 

 sentative of the latter group of lichens, viz. Usnea barbata. 



We may use either fresh material, or, as I have found suitable, 

 herbarium material softened in water. On submitting to micro- 

 scopic examination a delicate transverse section of a strong branch 

 of the thallus, we at once see that it is differentiated into pith and 

 cortex. The elements of both layers are much branched fungal 

 hyphae. The cortex, and also an axile strand of the pith, are dense 

 in structure, while the peripheral region of the pith is composed 

 of hyphae loosely arranged, and with air-containing spaces between 

 them. The green algae are easily seen at the boundary between 

 pith and cortex. They form there a special zone, which is every- 

 where traversed by hyphae running from pith to cortex. If we 

 desire to investigate the structure of other lichens, it will be of 



