18 



(c) The Hyphae are elongated, greatly-interlaced, 

 tube-like structures. Their walls are enormously 

 thick when compared with the size of the cavities. . 



3. PHYSIOLOGY. 



(a) Respiration. Oxygen is absorbed, and carbon 

 di-oxide liberated as in the higher plants. 



(b) Assimilation. The gonidia, as they contain 

 chlorophyll, decompose Carbon di-oxide and water, 

 and build up starch, at the same time liberating 

 Oxygen. This can only go on in the presence of 

 sunlight. 



The hyph?e fit closely to the gonidia, and carry to 

 them the crude sap which contains in solution 

 Nitrate of Potassium, Sulphates of Calcium and 

 Magnesium, and Chloride of Iron. These are all 

 obtained from the soil or rock on which the lichen 

 is flourishing. In return the hyphee receive a share 

 of the organic material elaborated by the gonidia. 

 Gonidia and Hyphae are thus closely associated and 

 are dependent on each other ; the gonidia receive 

 from the hyhpae the crude material, the hyphae 

 receive from the gonidia elaborated material. This 

 mutual give and take is called Symbiosis, and is 

 nowhere else exemplified in the vegetable kingdom. 



4. MODE OF GEOWTH. 

 The mode of growth, external structure, and 



