26 Practical Plant Biology. 



temperature round which growth is most vigorous is called the 

 optimum temperature. At certain upper and lower limits growth 

 ceases and finally death supervenes. The upper limit where death 

 supervenes is, for ordinary yeast, about 70 C. The spores, 

 however, may resist a higher temperature and this higher limit of 

 the spores may be further raised by desiccation. Dried spores may 

 actually grow after being exposed to a temperature of 120 C. 

 This greater resistance of the desiccated spores is probably con- 

 nected with the fact that desiccated proteins resist coagulation 

 while imbibed proteins at the same temperature are coagulated. 



The enlargement of existing cells and the development of new 

 ones require material. New wall substance cellulose, new proto- 

 plasm, new vacuoles must be formed. The crude material to be 

 converted into these structures must be available. This material 

 must be supplied in the medium in which the plant grows. In 

 breweries this medium is the sweet wort composed of the 

 germinated barley seeds. It may be regarded as a mixture of 

 proteins, carbohydrates and salts. These are modified and ab- 

 sorbed by the yeast plants and converted into the materials needed 

 for growth. 



Such conversion is called assimilation. 



PRACTICAL WORK. 



Mix up a i per cent, sol of gelatine (swelling the sheet gelatine first in 

 cold water). Observe it by transmitted light, and in a concentrated beam of 

 light. Compare the behaviour of a salt solution. Mix up a 10 per cent, sol 

 of gelatine. Observe how the sol sets to a gel when the temperature falls. 

 On a rise of temperature the sol state is again assumed. 



Shake the gel in a test tube and notice its elasticity. Violent shaking may 

 fracture the gel. When the fractured surfaces come together again the 

 fracture is healed. 



Shake up a little oil of cloves in water in a test tube. An emulsion results 

 which is whitish in colour. When left to stand the droplets run together and 

 rise as larger drops to the surface leaving the clear oil floating on the clear 

 water. When the oil is dispersed as droplets in'the water it scatters the light 

 penetrating the mixture and the latter looks white. 



Now dissolve 2 drops of oil of cloves in 10 c.c. of spirit. Add the solution 

 to 300 c.c. of water. Notice the appearance of a very faint bluish coloration. 

 The mixture looks clear to transmitted light, but a concentrated beam passing 

 into it appears brilliant blue. The droplets of oil are here reduced to the 

 dimensions of the colloidal state. 



Shake up some yeast in a little sugar-solution and make a suspension. 

 Notice the production of gas in the suspension and the smell of beer which 

 develops. Mount a drop of the suspension on a slide. Draw carefully an 

 ovoid cell showing the protoplasm and the vacuole. Draw cells showing the 

 different stages in the development of a cell by budding. Mount some yeast 

 in a 10 per cent, salt solution and observe plasmolysis. Crush a preparation 

 of yeast and look for emptied cell- walls. 



