3. BREATHING PORES. 



There are various structural adaptations met with among lichens, 

 whose special function is that of aeration, that is, of facilitating the 

 exchange of gases resulting from the processes of assimilation. Some 

 of these adaptations are readily visible to the naked eye, such as the 

 pores in some of the Cladonias and other fruticose lichens, also the 

 inflated thalli of some Parmelias. We have already shown that the 

 outer cortical layer of the lower fruticose lichens is frequently incom- 

 plete, thus making it possible for the assimilating algae to come in 

 contact with the necessary CO 2 of the atmosphere. The absence of 

 the lower cortical layer in many foliose lichens permits the entrance 

 of air. It is evident that thalli with both upper and lower cortical lay- 

 ers must have special structural adaptations to facilitate the exchange 

 of gases ; these we shall now briefly discuss. 



On careful examination of a thin vertical section of a foliose 

 thallus it is found that the cortical layers are not continuous. The 

 upper cortical layer, especially, contains intercellular canals or pores, 

 which open above to the exterior and below into the algal and medul- 

 lary tissues. These canals are especially numerous in the thin 

 areas of the upper cortical layer ; they are considerably branched 

 and pass upward or outward until they arrive at the epidermal layer, 

 where they assume a horizontal course, because the cells of this 

 tissue are elongated horizontally. In the dry state the cell-walls of 

 the cortical cells become shrunken, allowing the angles to come in 

 contact, thus closing the pores and reducing the loss of moisture to 

 a minimum. With the increase of moisture the cell-walls become 

 swollen and the intercellular canals enlarged, thus enabling the as- 

 similation of CO 2 to take place. These canals, which are desig- 

 nated breathing pores, occur in Nephromium, Solorina, Parmelia 

 and other genera. They occur even in the upper cortical layer 

 when no -lower cortical tissue is present. As a rule, however, they 

 are wanting in foliose lichens with only one cortical layer. Again 

 they may occur in both upper and lower cortical layers. In Sticta 

 and Stictina they predominate in the lower cortical layer. Their re- 

 lation to the cyphellae will be explained in the discussion of these 

 organs. 



4. CYPHELLAE. 



The structures known as cyphellae have long been familiar to 

 lichenologists. Haller (1776) was the first author who called atten- 



