374 A TEXTBOOK OF THEORETICAL BOTANY 



of the perithecium, and from its base asci containing ascospores are developed 

 in the usual manner. The spores are unicellular at first, but later become 

 septate. They often germinate while still in the perithecium, and the resulting 

 hyphal network is later expelled through the ostiole and floats freely in the 

 water. Such a structure doubtless serves to entrap free-floating cells of 

 Pleurococcus, and it may be a specialization to ensure the union of the two 

 constituents. 



Distribution 



Two factors have to be taken into consideration in relation to Lichen 

 distribution. In the first place there is the peculiar symbiotic life, which 

 we have already referred to, and secondly there is the method of dispersal. 



The ascospores are purely fungal structures, so that the possibility of 

 their producing a new Lichen of the same species depends on their encounter- 

 ing the requisite Alga. Lichens, however, also spread by means of small 

 portions of the thallus, which we have described as soredia, while in some 

 Lichens small nodular excrescences become detached and blown about 

 by the wind, as, for example, in Lecanora escu/enta, the so-called Alanna 

 Lichen (Fig. 360). In these ways both constituents are dispersed together, 

 and there is no doubt that in many species these are the most efficient means 

 of dispersal. 



Geographically Lichens are one of the most widely distributed groups of 

 plants, many genera and some species having an almost world-wide range. 

 In cold climates they are predominantlv, though by no means exclusively, 

 saxicoles, that is, rock or stone-livers, while in the tropics, on the other 

 hand, corticoles, or bark-livers, predominate and may seriously encumber 

 the trunks and branches of trees. Individuals may be more luxuriant in 

 warm climates, but the number of species is much greater in cold regions, 

 and indeed Lichen growth becomes more and more prominent in the vegeta- 

 tion as one travels away from the tropics. In the far north, beyond the 

 geographical limit of forests, lies the tundra, an immense stretch of heath- 

 like country where, in many places, Mosses and Lichens form the only 

 covering. Here the Reindeer Moss {Cladonia rangiferina) is one of the chief 

 Lichens (Fig. 361). It is a fruticose Lichen which is eaten as herbage by 

 the reindeer herds, and it is collected into barns by the Lapps for winter 

 forage. 



Lichens may claim to be found farther north and farther south and at 

 higher altitudes on the earth's surface than any other plants. They are 

 essentially pioneers. On fresh rock surfaces Lichens are the first plants 

 to appear. The humus which they accumulate becomes a breeding ground 

 for the spores of Mosses, and this leads on eventually to colonization by 

 flowering plants. The crustaceous forms are usually the earliest to appear 

 and they are followed by foliaceous types. The fruticose forms are usually 

 the culminating development. At each stage certain species are commonly 

 found together, so that Lichen " associations " may be recognized, like 



