I40 MORPHOLOGY 



occur on the lower surface of the lichen, correspond to those of the upper 

 in general structure, but there is no intermixture of thalline gonidia. That 

 Nostoc cells can grow and retain the power to form chlorophyll in adverse 

 conditions was proved by Etard and Bouilhac^ who made a culture of the 

 alga on artificial media in the dark, when there was formed a green pigment 

 of chlorophyll nature. 



Endotrophic cephalodia occur in many groups of lichens. Hue^ states 

 that he found them in twelve species of Aspicilia. As packets of blue-green 

 algae they are a constant feature in the thallus of Solorinae. The species of 

 that genus grow on mossy soil in damp places, and must come frequently 

 in contact with Nostoc colonies. In Solorina crocea an interrupted band of 

 blue-green algae lies below the normal gonidial zone and sometimes replaces 

 it — a connecting structure between cephalodia and a true gonidial zone. 



c. PsEUDOCEPHALODiA. Under this section have been classified those 

 cephalodia that are almost independent of the lichen thallus though to some 

 extent organically connected with it, as for instance that oi Lecidea panaeola 

 which originate on the hypothallus of the lichen and maintain their position 

 between the crustaceous granules. 



The cephalodia of Lecanora gelida, as described by Sernander',' might 

 also be included here. He watched their development in their native habitat, 

 an exposed rock-surface which was richly covered with the lichen in all 

 stages of growth. Two kinds of thallus, the one containing blue-green algae 

 {Chroococcus), the other bright-green, were observed on the rock in close 

 proximity. At the point of contact, growth ceased, but the thallus with 

 bright-green algae, being the more vigorous, was able to spread round and 

 underneath the other and so gradually to transform it to a superficial flat 

 cephalodium. All such thalli encountered by the dominant lichen were 

 successively surrounded in the same way. The cephalodium, growing more 

 slowly, sent root-like hyphae into the tissue of the underlying lichen, and 

 the two organisms thus became organically connected. Sernander considers 

 that the two algae are antagonistic to each other, but that the hyphae can 

 combine with either. 



The pseudocephalodia of Usnea species are abortive apothecia ; they are 

 surrounded at the base by the gonidial zone and cortex of the thallus, and 

 they contain no foreign gonidia. 



E. AUTOSYMBIOTIC CEPHALODIA 



Bitter* has thus designated small scales, like miniature thalli, that develop 

 constantly on the upper cortex of Peltigera lepidophora, a small lichen not 

 uncommon in Finland, and first recorded by Wainio as a variety of Peltigera 



1 Etard and Bouilhac 1898. 2 j^ue 1910. 3 Sernander 1907. * Bitter 1904. 



