STRUCTURES PECULIAR TO LICHENS 141 



canina. The alga contained in the scales is a blue-green Nostoc similar to 

 the gonidia of the thallus. Bitter' described the development as similar to 

 that of the cephalodia of Peltigera aphthosa, but the outgrowths, being lobate 

 in form, are less firmly attached and thus easily become separated and dis- 

 persed ; as the gonidia are identical with those of the parent thallus they 

 act as vegetative organs of reproduction. 



Bitter's work has been criticized by Linkola^ who claims to have dis- 

 covered by means of very thin microtome sections that there is a genetic 

 connection between the scales and the underlying thallus, not only with the 

 hyphae, as in true cephalodia, but with the algae as well, so that these out- 

 growths should be regarded as isidia. 



In the earliest stages, according to Linkola, a small group of algae may 

 be observed in the cortical tissue of the Peltigera apart from the gonidial 

 zone and near the upper surface. Gradually a protruding head is formed 

 which is at first covered over with a brown cortical layer one cell thick. The 

 head increases and becomes more lobate in form, being attached to the thallus 

 at the base by a very narrow neck and more loosely at other parts of the 

 scale. In older scales, the gonidia are entirely separated from those of the 

 thallus, and a dark-brown cortex several cells in thickness covers over the 

 top and sides ; there is a colourless layer of plectenchyma beneath. At this 

 advanced stage the scales are almost completely superficial and correspond 

 with the cephaloidal rather than with the isidial type of formation. The 

 algae even in the very early stages are distinct from the gonidial zone and 

 the whole development, if isidial, must be considered as somewhat abnormal. 



3. SOREDIA 



A. Structure and Origin of Soredia 



Soredia are minute separable parts of the lichen thallus, and are com- 

 posed of one or more gonidia which are clasped and surrounded by the 

 lichen hyphae (Fig. 80). They occur on the sur- 

 face or margins of the thallus of a fairly large 

 number of lichens either in a powdery excrescence 

 or in a pustule-like body comprehensively termed 

 a "soraHum" (Fig. 81). The soralia vary in form rig. 80. Soredia. a, of Physcia 

 and dimensions according to the species. Each ptdveruimtai^^\.-b, aiRama- 



■ , hnafarinacea Ach. x ooo. 



individual soredium is capable of developing into 



a new plant; it is a form of vegetative reproduction characteristic of lichens. 



Acharius' gave the name " soredia " to the powdery bodies with reference 



to their propagating function ; he also interpreted the soredium as an " apo- 



thecium of the second order.'' But long before his time they had been 



^ Bitter 1904. ^ Linkola 1913. ^ Acharius, 1798, p. xix, and i8io, pp. 8 and 10. 



