Lecture III. 25 



mass of the particles is sufficiently minute to be appreciably 

 disturbed by these impacts. It is called Brownian motion. 



The cell-wall may be rendered more plain if some yeast is 

 mounted in a 10 per cent, solution of common salt. The proto- 

 plasm is then driven in from the walls by the osmotic pressure of 

 the salt solution and the cell-wall appears standing out alone, and 

 its clear colourless nature may be observed. Again, if the cover- 

 glass of a preparation of yeast is pressed firmly and ground to 

 and fro on the slide, some of the cells will be ruptured and the 

 more or less completely emptied cell-walls may be observed 

 in the preparation. Their toughness and elasticity may be 

 observed. 



While the majority of the yeast plants are very uniform in size, 

 some cells in the suspension will be found which are much smaller 

 than the majority : also some of the larger cells will be found 

 with these smaller cells in contact with them. By observing a 

 number of these small groups it will soon appear that a smaller 

 cell arises as a minute lump or excrescence on a mature cell. 

 This swelling grows bigger and bigger until it attains the size of 

 the supporting cell. While the growing cell or bud increases in 

 size, its place of attachment remains small, and at some time in 

 its growth the protoplasm in the bud is cut off from the original 

 cell by the completion of the cell-wall. In this stage the bud is 

 very easily detached and is set free by any disturbance in the 

 medium. When the medium is undisturbed many generations of 

 buds may adhere together. In a solution kept at 30 C. it is 

 observed that about six hours elapse between the beginning and 

 the maturity of a budded cell". This budding is a most prolific 

 method of reproduction. 



Occasionally one finds in yeast preparations an unbroken cell- 

 wall containing three or four smaller complete cells. By special 

 methods of culture this type may be produced at will. The inner 

 cells are formed by the breaking up of the protoplasm of an original 

 cell into a small number of masses and each of these masses be- 

 coming covered with a cell-wall. Each of these enclosed cells is 

 called a spore. It is capable of becoming a new plant. 



Yeast surrounded with a sugar solution and kept at a tempera- 

 ture of about 30 C. grows rapidly. The individual cells multiply 

 and the buds grow quickly to maturity. Its rapid growth is 

 marked with the formation of bubbles of carbon dioxide and the 

 production of alcohol. If the temperature is lowered the rate of 

 growth is retarded, until at about 8 or 10 C. it ceases. Above 

 30 C. growth falls off and stops at about 60 or 70. The 



