FIN K—THE LICHENS OF MINNESOTA. 29 
perhaps of the same species, varying in size because of a difference in 
nutrition; but it is by no means certain that the thecial alge are 
derived from the thallus of the lichen in which they occur. Among 
the Minnesota lichens they may be looked for especially in mature 
plants of Dermatocarpon and Endocarpon, though they may occur 
in some other closely related types. The algal cells are usually 
smaller than those of the thallus, and are often found clinging to the 
asci or to the paraphyses. By some it is supposed that they are dis- 
persed with the spores and are at hand when the spores germinate, so 
that the symbiotic relation may be established at once and a thallus 
readily built up, provided 
other conditions are favor- fe 
able. However, it may well 
be doubted whether repro- 
duction often takes place in 
this way in nature, and it is 
much more probable that 
the main function of these 
algal cells is to nourish the 
fungal tissues within the 
apothecium. The fact that 
thecial algee are more com- 
mon inimmature than in ma- Fic. 6.—Endocarpon pusillum, a, The hymenial alge inter- 
ture apothecia would favor mingled with the asci and the paraphyses; 6, two germi- 
this view. They are found = yive Spores eee menial algwe. Enlarged 
in many immature apothecia 
of other lichen genera, but seldom persist. Doubtless these nonper- 
sisting thecial alge at least are foreign organisms which, having 
gained access to the young apothecia, endure for a time under more 
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G5; 
Bi 
SE: 
5 
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| 
or less unfavorable conditions. 
DEVELOPMENT OF THE APOTHECIA. 
This subject has been studied by various observers in Europe and 
America, and the origin and development seems to be very similar to 
that in the Ascomycetes in general. The first thing to be observed 
in the development of an apothecium is a mass of closely interwoven 
hyphe below the algal zone, in the medullary area—this, of course, in 
those lichens in which there is a distinct differentiation into thallus 
layers. In Collema and some related forms, this differentiation is 
wanting, but the development here also begins within the thallus. 
‘The crustose lichens, which are without the layers, have not been 
carefully studied as to apothecial development. 
The mass of hyphe constitutes the beginning of the development 
of the hypothecium, and the structure increases rapidly in size, 
spreading out laterally and also increasing in thickness. From the 
