148 MORPHOLOGY 



that, in many cases, the loosely attached soredia coating some of the 

 Cladonia podetia are of external origin, carried thither by the air-currents. 

 Insects too aid in the work of dissemination: Darbishire^ has told us how 

 he watched small mites and other insects moving about over the soralia of 

 Pertusaria amara and becoming completely powdered by the white granules. 



Darbishire' also gives an account of his experiments in the culture of 

 soredia. He sowed them on poplar wood about the beginning of February 

 in suitable conditions of moisture, etc. Long hyphal threads were at once 

 produced from the filaments surrounding the gonidia, and gonidia that had 

 become free were seen to divide repeatedly. Towards the end of August of 

 the same year a few soredia had increased in size to about 450/i in diameter, 

 and were transferred to elm bark. By September they had further increased 

 to a diameter of 520/1., and the gonidia showed a tendency towards aggre- 

 gation. No further differentiation or growth was noted. 



More success attended Tobler's'' attempt to cultivate the soredia of 

 Cladonia sp. He sowed them on soil kept suitably moist in a pot and after 

 about nine months he obtained fully formed squamules, at first only an iso- 

 lated one or two, but later a plentiful crop all over the surface of the soil. 

 Tobler also adds that soredia taken from a Cladonia, that had been kept for 

 about half a year in a dry room, grew when sown on a damp substratum. 

 The algae however had suffered more or less from the prolonged desiccation, 

 and some of them failed to develop. 



A suggestion has been made by Bitter" that a hybrid plant might result 

 from the intermingling of soredia from the thallus of allied lichens. He 

 proposed the theory to explain the great similarity between plants of Par- 

 melia physodes and P. tubulosa growing in close proximity. There is no 

 proof that such mingling of the fungal elements ever takes place. 



D. Evolution of Soredia 



Soredia have been compared to the gemmae of the Bryophytes and also 

 to the slips and cuttings of the higher plants. There is a certain analogy 

 between all forms of vegetative reproduction, but soredia are peculiar in 

 that they include two dissimilar organisms. In the Hchen kingdom there 

 has been evolved this new form of propagation in order to secure the con- 

 tinuance of the composite life, and, in a number of species, it has almost 

 entirely superseded the somewhat uncertain method of spore germination 

 inherited from the fungal ancestor, but which leaves more or less to chance 

 the encounter with the algal symbiont. 



From a phylogenetic point of view we should regard the sorediate lichens 

 as the more highly evolved, and those which have no soredia as phylo- 



' Darbishire 1907. 2 Tobler 1911^ 11. 3 Bitter 1901". 



