Interest in these minute “tubercles” and their enclosed “corpuscles” was revived by Itzigsohn[708] who examined them with an improved microscope. He macerated in water during a few days that part of the thallus on which they were developed, and, at the end of the time, discovered that the solution contained large numbers of motile bodies which he naturally took to be the corpuscles from the broken down tubercles. He claimed to have established their function as male motile cells or spermatozoa. The discovery seemed not only to prove their sexual nature, but to link up the reproduction of lichens with that of the higher cryptogams. The tubercles in which the “spermatozoa” were produced he designated as antheridia. More prolonged maceration of the tissue to the very verge of decay yielded still larger numbers of the “spermatozoa” which we now recognize to have been motile bacilli.
Tulasne[709] next took up the subject, and failing to find the motile cells, he wrongly insisted that Itzigsohn had been misled by mere Brownian movement, but at the same time he accepted the theory that the minute conceptacles were spermogonia or male organs of lichens. He also pointed out that their constant occurrence on the thallus of practically every species of lichen, and their definite form, though with considerable variation, rendered it impossible to regard them as accidental or of no importance to the life of the plant. He compared them with fungal pycnidia such as Phyllosticta or Septoria which outwardly they resembled, but whereas the pycnidial spores germinated freely, the spermatia of the spermogonia, as far as his experience went, were incapable of germination.
C. Occurrence and Distribution
a. Relation to Thallus and Apothecia. We owe to Tulasne[710] the first comparative study of lichen spermogonia. He described not only their outward form, but their minute structure, in a considerable number of representative species. A few years later Lindsay[711] published a memoir dealing with the spermogonia of the larger foliose and fruticose lichens, and, in a second paper, he embodied the results of his study of an equally extensive selection of crustaceous species. Lindsay’s work is unfortunately somewhat damaged by faulty determination of the lichens he examined, and by lack of the necessary discrimination between one thallus and another of associated and intermingled species. Both memoirs contain, however, much valuable information as to the forms of spermogonia, with their spermatiophores and spermatia, and as to their distribution over the lichen thallus.
Though spermogonia are mostly found associated with apothecia, yet in some lichens, such as Cerania (Thamnolia) vermicularis, they are the only sporiferous organs known. Not unfrequently crustaceous thalli bear spermogonia only, and in some Cladoniae, more especially in ascyphous species, spermogonia are produced abundantly at the tips of the podetial branches ([Fig. 109]), while apothecia are exceedingly rare. Usually they occur in scattered or crowded groups, more rarely they are solitary. Very often they are developed and the contents dispersed before the apothecia reach the surface of the thallus; hence the difficulty in relating these organisms, since the mature apothecium is mostly of extreme importance in determining the species.
Fig. 109. Cladonia furcata Schrad. Branched podetium with spermogonia at the tips (after Krabbe).
Fig. 110. Physcia hispida Tuckerm. Ciliate frond. a, spermogonia; b, apothecia. × ca. 5 (after Lindsay).
In a very large number of lichens, both crustaceous and foliose, the spermogonia are scattered over the entire thallus ([Fig. 110]), covering it more or less thickly with minute black dots, as in Parmelia conspersa. In other instances, they are to some extent confined to the peripheral areas as in Parmelia physodes; or they occur on the extreme edge of the thallus as in the crustaceous species Lecanora glaucoma (sordida). In Pyrenula nitida they grow on the marginal hypothallus, usually on the dark line of demarcation between two thalli.