118 PHACTICAL BOTANY. 



to a definite zone : this is thus an example of the homo 6mer oils 

 Lichens. 



III. Some specimens of Parmelia will be found to 

 show more closely packed convolutions than others, and, 

 on these it may be noted that apothecia are few, or 

 entirely absent: it is on these that the soredia are 

 more especially to be sought for. Soak such a specimen 

 (which has previously been kept dry) in water : then, 

 having dried off the excess of water with blotting-paper, 

 press its upper surface on a glass slide, when, on removing 

 it again, a sediment will be left in the water on the 

 slide : mount this in a drop of glycerine, warm gently, 

 and examine under a high power. Various objects 

 which have no relation to the thallus will be seen, such 

 as grits, various Algae, &c. &c. Amongst these will be 

 seen roundish bodies (soredia) of various size and 

 complexity, composed of the same constituents as the 

 thallus, viz. 



a. Fungal hyphse, enveloping, and completely 

 inclosing 



6. The gonidia, of which one or more may be present 

 in each soredium. 



Attempts should he made to grow Lichens from the soredia : 

 pieces of porous tile should be heated to kill other organisms, then 

 saturated with water, and the soredia sown in small numbers on 

 their surface. 



In some other Lichens the soredia are produced in a more 

 prominent manner than in Parmelia: thus in Usnea and Cladonia 

 they may be recognised as a powdery covering of some parts of 

 the thallus, and are especially obvious after rain. In the gelatinous 

 Lichens soredia are absent, but the thallus may be reproduced by 

 the outgrowth and ultimate abstriction of processes consisting of 

 both Algal and Fungal constituents : these may often be seen 

 projecting from the outer surface in sections of these Lichens. 



