368 A TEXTBOOK OF THEORETICAL BOTANY 



or it may develop only around the upper parts. For details of the sexual 

 organs see p. 372. 



Ascospores 

 Algal cells 



Fig. 353. — rXanthoria parietiuo. Vertical section through 

 part of an apothecium showing thallus tissue below, 

 which forms the margin of the apothecium, and 

 the hymenial layer containing bilocular ascospores. 



Nutrition 



In the study of the Lichens special interest is attached to their physiology. 

 In the higher plants food is derived partly from the air in the form of Carbon 

 dioxide and partly from the soil as inorganic compounds. In the Fungi 

 we know that owing to the absence of chlorophyll the organic nutrients cannot 

 be manufactured, and the Fungus becomes either a parasite or a saprophyte 

 in order to overcome this deficiency. The Lichens, owing to symbiosis, are 

 enabled to obtain their food like a flowering plant or, for that matter, like an 

 Alga. In the Lichen thallus the algal cells obtain protection and moisture 

 in the thallus of which they form a part and enjoy conditions under which 

 they can grow rapidly. Probably they assimilate an excess of Carbon dioxide 

 and provide the Fungus with carbohydrates which it uses in its nutrition. 

 The Fungus undoubtedly also consumes the material of dead gonidia and 

 may sometimes invade the living cells. 



It is interesting to note that the Lichens are not the only organisms that 

 have evolved this type of symbiosis. There are several cases in the animal 

 kingdom where algal cells are made use of to provide free carbohydrates. 

 In the common Hydra viridis we have a simple organism somewhat resembling 

 a minute sea-anemone in which the outer tissues of the body contain algal 

 cells of the genus Chlorella (Fig. 354). Another example is found in one of 

 the Nematode worms, Convoluta roscojfensis. In this example it has been 

 shown that the worm is unable to live in the absence of its algal symbiote. 



In many Lichens there is found to be abundant excretion of Calcium 

 oxalate, which is laid down on the outside of the fungal hyphae. In Lecanora 



