128 THE COLOURLESS PLANT CELL. THE YEAST PLANT 



world. All of these colourless plants, with the excep- 

 tion of a few kinds of bacteria, obtain the materials 

 for the formation of new protoplasm and the energy 

 for carr57ing out their life processes by absorbing liquid 

 organic substeinces from outside. 



Yeast (Saccharomyces '). — A simple organism with 

 which it is convenient to begin the study of colourless 

 plants is the unicellular yeast plant. There are many 

 different kinds or species of yeast, some of them differ- 

 ing markedly in the appearance and size of the cell, 

 but mainly distinguished by the difference of their 

 activities. There is convincing evidence that the 

 yeasts are derived from a certain group of the higher 

 fungi, but they live and maintain themselves indefinitely 

 as unicellular plants. 



Structure (Fig. 14). — The single yeast cell is spherical 

 or oval in shape, about 8 to 12 ju, in diameter — a small 

 cell on the scale of the tissue cells of the higher plants, 

 but about the same size as a cell of Protococcus — sur- 

 rounded by a cellulose cell wall. The cytoplasm is 

 more or less granular, and there is a central oval vacuole, 

 though the diameter of this in proportion to that of 

 the whole cell is not so great as in the ordinary adult 

 tissue cell of the higher plant — ^in other words, the 

 cytoplasmic layer lining the cell wall is relatively 

 thick (Fig. 14, C). The central vacuole is really intra- 

 nuclear — fine threads of chromatin apphed to the 

 inside of nuclear membrane extending around the 

 vacuole from an aggregation of chromatin at one end 

 (Fig. 14 C, chr.). This aggregation of chromatin can 

 sometimes be seen as a granule in the living cell. 

 According to the metaboUc condition of the cell, other 

 small vacuoles or droplets of liquid, and granules of 



' " Sugar-fungus."' 



