16 The Status of the Al go- Lichen Hypothesis. 
" Parasite " and " Host," and insist that there can be no such a 
thing as a ''mutual benefit" parasitism in nature as is claimed to 
be present in case of the lichen-fungi and the algae. The latter ob- 
jection Sargent explains {Am. Mo. Mic. Jour., Feb., 1887) by say- 
ing that while the algae furnish the necessary nourishment for the 
fungus, the latter in turn protects the former from excessive dry- 
ness and sunshine, allowing only enough softened light as is neces- 
sary to decompose the carbon dioxide, and, by acting as a sponge, 
takes up water readily and retains it, thus insuring at least a mod- 
erate supply of water for the algae even in dry weather ; moreover, 
it is a well-known fact that fungi in growing give ofE carbonic diox- 
ide. This the lichen hyphae furnish to the algae, and they in turn 
give back oxygen, etc., to the hyphae. As to the fact that some 
lichens grow in comparatively dry places, he thinks that this is not 
a very serious objection, since in some lichens we have hymenial 
gonidia which are ejected together with the spores ; in others sore- 
dia, by means of which new plants can be formed without the aid of 
spores. Again, the species of algae supposed to act as gonidia are 
those species that have become adapted to the frequent dry spells 
incident to terrestrial life. He further insists that the differ- 
ences between the fungal-algal elements of a lichen and free-living 
fungi and algae are just those differences that would result from the 
parasitical relationship claimed by the dualists. 
Nylander says that in no case do the gonidia arise from the 
hyphae, but from the parenchymatous cortical cells observed by him 
in the prothalline filaments of germinating spores. Crombie for- 
merly held that the gonidia might come from the hyphse or the 
hyphae from the gonidia. Later, he says the gonidia are of thalline 
origin. He claims to have seen the germination of spores and growth 
of young lichen thalli on rocks, etc., where no alga3 or gonidia could 
be seen. At first only the young hyphae were seen. Later, go- 
nidia were found. These he believes to have originated in certain 
glomerules noticed on the young hypothallus. These glomerules 
he claims contain gonidia in various stages of development. They 
finally become thicker and form the cortical layer. He then uses 
Nylander's explanation as to the free state of the gonidia in the in- 
terior of the thallus: ''The cortical stratum gradually increasing and 
extending is at the same time dissolved (resorbed. physiologically 
speaking) beneath, and the gonidia consequently become free." 
Crombie says further that *' the contact between the hyphae and 
gonidia is in no way genetic or parasitic. . . . The gonidia are 
