INTRODUCTORY. 3 



functions, to ascertain what are the fundamental properties 

 of their protoplasm. 



An acquaintance with the fundamental properties of 

 protoplasm in virtue of which the processes of nutrition are 

 effected may be perhaps most readily acquired by making a 

 series of observations upon some small simple plant which 

 can be obtained in quantity and of which the structure is 

 known. The Yeast-plant (Saccharomyces Cerevisuz) may be 

 conveniently taken for this purpose. A drop of yeast placed 

 under the microscope will be found to contain a great number 

 of minute, more or less oval cells (Fig. 2, a), to the presence 



FIG. 2. a,, A Yeast-cell : b, a Yeast-cell budding. 



of which the turbidity of the liquid is due. In its structure 

 a yeast-cell resembles a resting Haematococcus-cell, but the 

 yeast-cell is much smaller and is uncoloured. If now a drop 

 of yeast be added to a quantity of a liquid known as Pasteur's 

 solution, which consists of distilled water holding a small 

 percentage of certain inorganic salts and a larger percentage 

 of certain organic substances in solution, it will be found 

 that the Pasteur's solution, which is clear at first, becomes 

 turbid, if allowed to stand for a few hours in a warm place. 

 A drop of it examined under the microscope will be seen to 

 contain a great- number of yeast-cells, and it will also be 

 seen that the cells are actively multiplying by the formation 

 of small outgrowths (Fig. 2, b\ which gradually enlarge until 

 they attain nearly to the size of the parent-cell, when they 

 become detached and constitute new individuals. It is 

 evident that an enormous multiplication of the yeast-cells 

 originally introduced into the Pasteur's solution has taken 

 place, and this necessarily implies that a considerable quantity 

 of protoplasm and of cell-wall has been formed. If, in a 



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