270 FOUNDATIONS OF BOTANY 



326. Multiplication of Yeast It is -worth -while to notice the 



fact that yeast is one of the fe-w cryptogams -which have for ages 

 heen largely cultivated for economic purposes. Very recently yeast 

 producing has become a definite art, and the cakes of compressed 

 yeast so commonly sold afford only one instance of the success 

 that has been attained in this process. While yeast-cells are under 

 favorable conditions for growth, they multiply -with very great 

 rapidity. Little protrusions are formed at some portion of the 

 cell-waU, as the thumb of a mitten might be formed by a gradual 

 outgrowth from the main portion. Soon a partition of cellulose 

 is constructed, which shuts off the newly formed outgrowth, making 

 it into a separate cell, and this in turn may give rise to others, 

 while meantime the original cell may have thrown out other ofi- 

 shoots. The -whole process is called reproduction by budding. It is 

 often possible to trace at a glance the history of a group of cells, 

 the oldest and largest cell being somewhere near the middle of the 

 group and the youngest and smallest members being situated around 

 the outside. Less frequently the mode of reproduction is by means 

 of spores, new cells (usually four in number), formed inside one of 

 the older cells (ascus). At length the old cell-waU bursts, and the 

 spores are set free, to begin an independent existence of their own. 



In examining the yeast-cell the student has been making the 

 acquaintance of plant life reduced almost to its lowest terms. The 

 very simplest plants consist, like the slime moulds, of a speck of 

 jelly-like protoplasm. Yeast is more complex, from the fact that its 

 protoplasm is surrounded by an envelope of cellulose, the cell-wall. 



THE STUDY OF PHYSCIA 



327. Occurrence. — Physcia is one of the commonest lichens. It 

 grows attached to the bark of various trees. 



328. The Thallus. — Physcia consists chiefly of an irregularly 

 expanded growth somewhat leaf-like in texture. It is best to be wet 

 for study. Is it separable from the bark to which it is attached or 

 is it combined with it (incrusted) ? Describe the general outline of 

 the margin, the general color, and any special variations of color 

 above, also below. How is the thallus attached to the bark ? 



