1903 Symbiosis 159 



with water and food-stuffs from the earth, whilst it receives 

 for these services various organic materials produced in the 

 green leaves of the flowering-plant. It is also a fact that 

 certain water-plants harbour in cavities of their stems and 

 leaves minute algae, but it does not seem quite clear what 

 advantage the larger plant reaps from its small lodger, who 

 evidently gets house-room without rent ! 



The relations between two cryptogamic plants associated 

 for mutual benefit is well exhibited by the complex plants 

 we call Lichens. Lichens are among the first vegetation 

 to appear upon the surface of rocks and stones, and find a 

 footing where more highly organised plants would perish. 

 They are seen clinging to rocks, walls, palings, and trees in 

 a great variety of shapes and colours. Some are mere layers 

 of flattened thallus like the yellow Parmelia, others form a 

 wavy thallus or become much branched like the reindeer 

 moss or the so-called cup-mosses of our shady banks and 

 commons. The Lichen consists of a distinct fungus and an 

 alga, one destitute of chlorophyll and forming the principal 

 structure of the plant-body, performing the processes of 

 reproduction and absorption ; the other possessing chloro- 

 phyll, even if disguised by other pigments, and capable of 

 the food-manufacture necessary for the composite organism. 

 A real and helpful partnership is, therefore, carried on. 

 The fungal portion is closely allied to the Ascomycetes, a 

 section of Fungi to which the Pezizas belong ; the algal 

 portion is composed, in different lichens, of a variety of 

 genera — Pleurococcus, Nostoc, &c. 



There are two methods by which the whole colony may 

 be reproduced — a purely vegetative method and a more 

 complex (supposed) sexual process. In the first way 

 minute particles of the lichen, looking like powdery dust, 

 are given off from the surface of the thallus or branches : 

 these consist of one or more algal cells enclosed in a web 

 of fungus-hyphae or threads, and are capable of developing, 

 under suitable conditions, into new plants. The other 

 process is said to be due to sexual fertilisation. There are 

 to be found on a mature lichen flat or depressed bodies, 

 called apothecia ; these bear on their concave surface a 

 layer of cells filled with flask-shaped organs, which when 



