FINK—THE LICHENS OF MINNESOTA. 31 
portion of the foliose lichens and a few fruticose and crustose ones 
seldom produce apothecia and spores. 
FRAGMENTATION. 
Another method of vegetative reproduction among lichens is by 
fragments of thalli becoming detached and blowing away and growing 
upon a suitable substratum. It does not matter how small the frag- 
ment provided it contains both the alge and the fungal hyphe. 
There may be all the layers of the thallus represented in the fragment 
or not. Sometimes the fragment is an isidioid branchlet and in other 
instances it is a large and conspicuous portion of the thallus. The 
latter condition is well illustrated in Usnea longissima, long masses of 
which are often seen in northern Minnesota hanging over branches, 
without any attachment whatever. In foliose and crustose lichens 
also the fragments may be large, and even whole thallij may be torn 
loose by the wind and transported to a new substratum and there grow. 
REJUVENESCENCE. 
In a considerable number of lichens the older portions of the thallus 
die while the younger portions continue to grow. This condition is 
quite common among Cladonias, where the basal portions of the 
podetia die and the branching above continues. The branches thus 
become separated and a number of ‘individuals arise from one. 
Usually such an assemblage forms a dense cluster, but the central 
and more raised portion of the cluster frequently dies or is blown away, 
leaving a continuous or more or less broken ring of plants. In foliose 
and crustose lichens the central portion of the thallus often dies and 
the outer portions form a ‘‘fairy ring.” The ring often becomes dis- 
continuous, and thus a number of individuals arise from one. The 
dying at the center may be due to age or to the exhaustion of food 
substances from the substratum. Lecanora muralis and Lecidea 
speirea are species of crustose lichens in which this method of repro- 
duction is often seen (pl. 33, facing p. 175). 
SPORES. 
The spores of lichens have repeatedly been proved to be capable 
of germination in cultures and of producing the usual forms of thalli 
of their species, both when sown with the alge and when sown with- 
out them in favorable nutrient media. But the question still remains 
whether lichens are often produced from spores in nature. [Evi- 
dently, except in the instances of germination in certain culture 
media, the spores of lichens must happen to come into contact with 
the alga of the same species as the one that forms the algal symbiont 
in the species of lichen by which the spore was produced. And, in 
addition, the conditions, substratic and other, must be sufficiently 
