XANTHIDIA. 

 X. 



[ 692 ] 



YEAST. 



XANTHTDIA. The bodies found in 

 flint, and thus called, are sporangia of Des- 

 midiaceze (PI. 19. figs. 22-28). They have 

 been distributed in genera and species, the 

 description of the characters of which would 

 be useless. 



XANTHIDIUM, Ehr. A genus of Des- 

 midiacese. 



Char. Cells single, constricted in the 

 middle; segments compressed, entire, spi- 

 nous, with a circular, usually tuberculated 

 projection near the centre. Spines more 

 than two to each segment. 



X. armatum (PL 10. fig. 23 ; fig. 24, empty 

 cell, showing the projections). Segments 

 broadest at the base; spines short, stout, 

 tri- or multi-fid. Length 1-180". 



X.fasciculatum (PL 10. fig. 25). Segments 

 with from four to six pairs of subulate mar- 

 ginal spines; central projections minute, 

 conical, and not beaded. Common. Length 

 1-400". 



Four other British species. 



BIBL. Ralfs, Brit. Desmid. 111. 

 o XANTHIOPYXIS, Ehr. A genus of 

 fossil Diatomaceae, consolidated with PYXI- 

 DICULA. It consisted of those species, the 

 margins of the valves of which are fur- 

 nished with a dentate membrane, or the 

 surface covered with setae or hair-like pro- 

 cesses. From Bermuda. 



BIBL. Ehrenberg, Ber. d. BerL Akad. 

 1844. 264; Kiitzing, Sp. Alg. 23. 



XENODOCHUS, Schlecht. An obscure 

 genus of Fungi, consisting of microscopic, 

 short, curved, usually shortly stipitate fila- 

 ments, attenuated at each end, composed of 

 a moniliform row (five to fifteen) of globose 

 cells filled with black granules ; formed in 

 the Uredo-frmts ofPhragmidium inerassatum, 

 /3 mucronatum, on Poterium. Placed among 

 the Puccinaei by Berkeley ; near Torula by 

 Fries. 



BIBL. Schlechtendahl, Linncea, i. p. 237. 

 pi. 3. fig. 3; Fries, Summa Veg. p. 505; 

 Berkeley, Ann. Nat. Hist. i. p. 263. 



XYLARIA, Schrank. A genus of Sphas- 

 riacei (Ascomycetous Fungi), several of which 

 are common on rotten wood, stumps of trees, 

 &c. They are branched, horny or fleshy 

 bodies, with often clavate lobes, whitish 

 and mealy when young, afterwards brown 

 or black, with black, horny, immersed peri- 

 thecia all over the branches, or with the tips 

 barren; the perithecia have a black centre 

 composed of asci, each containing eight 

 (usually uniseptate) spores. 



BIBL. Berk. Brit. Flor. ii. pt. 2. p. 234 

 (Nos. 8 to 11); Fries, Summa Veg. p. 381. 



Y. 



YEAST (-PLANT) .This well-known sub- 

 stance, which possesses the remarkable pro- 

 perty of resolving sugar in solution into 

 alcohol and carbonic acid, consists of a mi- 

 nute fungus, or rather of a particular condi- 

 tion of development of a certain fungus. 



When yeast from an actively fermenting 

 liquid is examined with the microscope, it is 

 seen to consist of myriads of minute cells or 

 vesicles, of about 1-3000 to 1-2400" (PL 20. 

 fig. 23) in diameter, containing a nucleus 

 and some granules. During the progress of 

 the fermentation, these cells increase in 

 number, by budding, until either the sugar 

 or the nitrogenous matter of the fermenting 

 liquid is exhausted, when the cells, especially 

 those nearest the surface, become elongated, 

 remaining connected end to end, until they 

 reach the surface, where they produce their 

 fructification. 



The growth of the yeast-plant has been 

 carefully studied by several observers. We 

 may describe some observations of our own, 

 which confirm those of Mitscherlich and 

 others. Some fresh wort, in which fermen- 

 tation had commenced, was obtained from a 

 brewery, and a drop of the liquid, containing 

 yeast-globules, placed upon a slide, and co- 

 vered with a piece of thin glass. After the 

 removal of the extraneous liquid, the upper 

 glass plate was cemented to the lower one ; 

 the slide was then placed under the micro- 

 scope, with the l-4th object-glass and the 

 micrometer eyepiece, in such a manner, that 

 several well-formed globules were visible, 

 and these were drawn on ruled paper. 



At first the globules or cells enlarged until 

 they had attained a certain size ; then there 

 elapsed a short interval, during which no 

 change was observable. Next there took 

 place a projection of some point of the cell- 

 wall, which first appeared as a little point- 

 like bud, afterwards becoming larger and 

 larger, until at last a new cell, of the size of 

 the parent-cell, was formed. Within three 

 hours, a cell was so far developed, that a 

 new one was formed from it, and thus an 

 independent individual perfectly developed. 

 The rapidity of growth probably varies with 

 the temperature and the nature of the 

 process; in twenty-four hours, when the 

 thermometer was at about 78 in the day, 

 sixteen cells were developed from one ; after 



