XYLEM. 



824 1 



YEAST. 



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, im- 

 mersed perithecia all over the branches, or 

 with the tips barren ; the perithecia have a 

 black centre composed of asci, each contain- 

 ing eight (usually uniseptate) spores. 



BIBL. Berk. Br. Fl ii. pt. 2. 234; Fries, 

 Sum. Teget. 381. 



XY'LEM. Nageli's name for the woody 

 tissue forming part of the fibro-vascular 

 bundles of stems. It is composed of spiral 

 vessels, woody fibres, pitted ducts, and 

 wood-parenchyma. 



X YLOG'RAPHA, Fr. A genus of Gra- 

 phidei, Lichenaceous Lichens. 



Char. Apothecia black, linear, plane, in- 

 ternally ashy ; spores 8, simple. 4 sp., rare. 

 (Leighton, Lich. Flora, 390.) 



Y. 



YEAST(-PLANT). This well-known 

 substance, which possesses the remarkable 

 property 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, about 1-3000 to 1-2400" (PI. 26. 

 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 eye-piece, 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 

 a time the growth slackened ; finally no fur- 

 ther increase took place, undoubtedly be- 

 cause all was removed from the liquid which 

 could serve for their growth. Growing glo- 

 bules from this experiment are figured in 

 PI. 26. fig. 23. 



Berkeley and Hoffman went a step further, 

 by contriving to get an atmosphere of air 

 round the drop of sugar and water, and 

 saw the cells of the yeast-fungus as soon as 

 it reached the air developed into a Peni- 

 cittium. 



By the observations of numerous compe- 

 tent investigators, it seems certain that the 

 fermentation of Deer, of wine, and in fact 

 all vinous fermentation, is effected by the 

 growth of this plant ; and after the evidence 

 brought forward in the articles FERMENTA- 

 TION, TORULA, and VINEGAR-PLANT, there 

 is little doubt that the Vinegar-plant, the 

 Oidium lactis, and other supposed distinct 

 plants are but forms of the Yeast-plant. 

 PI. 26. fig. 24 exhibits the condition of the 

 Yeast-plant on the surface of exhausted 

 wort of malt, before the Vinegar-fungus 

 appears ; fig. 756, page 773, the Torttla~ftirm 

 at the margins of the surface of liquids. 



We cannot clearly make out any difference 

 between the Hop yeast' and l bottom yeast' 

 (Oberhefe and Unterhefe of the Germans), 

 beyond the difference resulting from more 

 or less active development : when the 

 growth is rapid the cells are more spherical 

 and become quickly detached, and the evo- 

 lution of gas comes up more to the surface ; 

 when the yeast vegetates quietly at the 

 bottom of the liquid, its cells are more 

 elongated. We do not believe the yeast- 

 cells ever burst to discharge reproductive 

 granules. The globular form is known by 

 various names, as Mycoderma cerevisia, 



