_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. [Illustration: Fig. 109. _Cladonia furcata_ Schrad. Branched podetium with spermogonia at the tips (after Krabbe).] [Illustration: 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.