DEVELOPMENT AND REPRODUCTION OF LICHENS. 351 



Thallus are rather arranged in the form of filaments, which pene- 

 trate the superficial layers of the bark whereon such Lichens grow, 

 and which are sometimes also so interwoven at the outer surface 

 as to form a sort of cuticle. Interposed among the ordinary cells 

 of the Thallus we very commonly find certain green globular cells, 

 arranged in single bead-like filaments ; these, which are termed 

 Gonidia, being found to be capable of reproducing the plant when 

 detached, must be considered as yemmce. From the recent obser- 

 vations of various Botanists, and especially from those of Dr. 

 Hicks (p. 320), it appears that many of the forms which have been 

 ranked among Unicellular Algae, are in reality transitory conditions 

 of these Gonidia, which may multiply themselves by binary sub- 

 division to a vast extent, without any essential change in their 

 condition. It was long since observed by Mr. Thwaites (p. 319, 

 note), that interlacing filaments are sometimes found in the midst of 

 the intercellular substance which holds together the cells of masses 

 of Palmella ; and this seems to constitute a very definite approach 

 to the Lichenoid condition. For in the higher tribes of Lichens 

 we find the interlacing filaments forming a tough cortical envelope 

 to both surfaces ; whilst in the interior of the firm ' crustaceous ' 

 Thallus the gonidial cells are found in regular layers. Sometimes 

 these increase in particular spots, and make their way through the 

 upper cortical layer, so as to appear on the surface as little masses 

 of dust, which are called Soredia. 



262. Besides these, Lichens are believed to contain proper Genera- 

 tive organs, by which a true Sexual reproduction is effected. In 

 addition to the 'fructification,' which is commonly recognized by 

 its projection from the surface of the Thallus, the researches of 

 M. Tulasne have detected a set of peculiar organs of much smaller 

 size, not unlike the male Receptacles of Fuci {§ 258), to which he 

 has given the appellation of Spermoyonid. From the exterior of 

 the cellular filaments which line these cavities, a vast number of 

 minute oval bodies termed Spermatid are budded off, which, when 

 mature, escape in great numbers from the orifices of the spermo- 

 gonia. They differ from ordinary Antherozoids in being destitute of 

 any power of spontaneous movement,, and we cannot yet indubitably 

 assign to them the male character, although various considerations 

 concur to render their performance of this function highly probable. 

 The Female portion of the Generative apparatus, though some- 

 times dispersed through the Thallus, is usually collected into special 

 aggregations, which form projections of various shapes ; these, 

 although they have received a variety of designations according to 

 their particular conformation, may all be included under the 

 general term Apothecid. When divided by a vertical section, these 

 bodies at their maturity are found to contain a number of Asci or 

 spore-cases, arranged vertically in the midst of straight elongated 

 cells or filaments, which are termed Paraphyses. Each of the 

 asci contains a definite number of Spores (usually eight, but 



