REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



229 



represented in Jig. 150. The central portion 

 of the disc is lined by an ascophorous mem- 

 brane. The overhanging marginal fold still 

 exhibits the filaments bearing spermatia, which 

 characterised the earlier condition of the 

 receptacle. Our authors are inclined to admit 

 that the spermatia are to be considered as a 

 male product, and the whole organ as analo- 

 gous to a hermaphrodite inflorescence. The 

 relative positions of the spermatia and thecae 

 seem admirably adapted to insure their con- 

 tact with each other. 



44. We now pass to the Lichens, with re- 

 spect to which the greater part of our informa- 

 tion is again owing to the researches of MM. 

 Tulasne.* In these plants, as in the Fungi, the 

 germination of the spore consists in the 

 emission of a hollow filament from some part 

 of its surface. This filament which is simply an 

 extension of the spore-membrane, branches 

 repeatedly and spreads over the surface, on 

 which the spore has been sown ; at the 

 same time it divides by numerous septa, 

 which occur at irregular intervals. By the 

 intertwining of the resulting ramifications, a 

 stroma is formed, to which the term hypo- 

 thallus is applied, and which constitutes the 

 vegetative system of the future lichen. So 

 far the development is the same as the Fungi. 

 At a longer or shorter period after the forma- 

 tion of the hypothallus, we begin to observe 

 upon its surface a whitish layer of spheroidal 

 cellules, intimately united with each other, as 

 well as with the filaments from which they 

 take their origin. This layer serves as the 

 groundwork for a second formation of glo- 

 bular cells. These are distinguished from their 

 predecessors, as well by the regularity of their 

 form, as by the granules of chlorophylle 

 which they contain. They are called gonidia, 

 and are peculiar to the Lichens, among which 

 their occurrence is almost constant. 



45. Such is the origin of the thallus, which, 

 although, at first sight, it appears to consti- 

 tute the whole plant, forms only a part, and 

 that not the most essential part of its vege- 

 tative system. In the Verrucariae, the most 

 simply organised Lichens with which we are 

 acquainted, it does not attain to any higher 

 development than that above described. 

 The receptacles (apothecia), which closely 

 resemble those of a Sphoeria, are formed 

 upon the surface of the hypothallus, which can 

 only be distinguished from the stroma of 

 the Fungus by the presence of scattered col- 

 lections of gonidia. In the more complicated 

 fpliaceous Lichens, such as Parmelia, the 

 mature thallus is formed of two kinds of 

 tissues, the medullary and the cortical. The 

 cortical tissue forms two layers an inferior 

 and superior and consists of thick-walled 

 cells intimately adherent to each other, and 

 resembling those of analogous structure, 

 which so often form the peridia of the higher 



* Mem. pour servir a 1'Histoire organographique 

 et phvsiologique des Lichens, Ann. des Sc Xat. 

 3me s" t. xvii. pp. 5. and 173. 



Fungi. From the surface of the inferior layer 

 are given off numerous laminar root-like 



Fig. 151. 



Vertical section of the apothecium of a Lichen (Par- 

 melia aipolia) and of the subjacent tissue of the 

 thallus, 200 diam. 



a, Lamina proligera, consisting of thecae and para- 

 physes ; 6, tissue of thick- walled cells continuous 

 with the cortical tissue of the thallus. Subjacent 

 to this, but separated by an irregular line of 

 gonidia, is the medullary" filamentous layer. 



appendages: The medullary substance con- 

 sists of a filamentous central layer, the 

 elements of which resemble those of the 

 hypothallus, and are directly continuous with 

 them ; on either side of this layer, between 

 it and the cortex, or rather embedded in its 

 substance, are the gonidia, which form a green 

 tissue, distinguishable by the naked eye. 

 To these a special function is assigned, which 

 shall be noticed at the conclusion of the 

 article under the head of Gemmation. 



46. We have next to describe the recepta- 

 cles, within or upon which the spores or spore- 

 like organs (spermatia and stylospores) are 

 produced. Of these there are three varieties, 

 to which the terms apothecia, spermogonite, 

 and pycnides, have been applied. The most 

 common form of the apothecium is that of a 

 disc, which may be plane, convex or cup- 

 shaped. This form is that which charac- 

 terises the gymnocarpons Lichens. In the 

 Angiocarpea, the organ is closed upwards, 

 its superior surface becoming internal, so as 

 to form a conceptacle like that of the Pyreno- 

 mycetes, the form of which is subject to 

 considerable variation. In either case, it is 

 composed of two layers, the inferior or ex- 

 ternal, being formed of thick-walled cells 

 which are soldered together, and resemble 

 those of the epidermal layers of the thallus. 

 The superior or internal layer is called the 

 lanwia proligera. It is formed of two kinds 



Q 3 



