2 VEGETABLE PHYSIOLOGY 



and are produced in large numbers. In other cases the 

 little mass of living substance is not capable of locomotion, 

 but may be found floating about in water, or enclosed in 

 particular cavities in its parent plant. 



The jelly-like substance of which these bodies are com- 

 posed is living and capable of carrying out all the functions 

 necessary for its life, growth, and multiplication. It is 

 called protoplasm, and each portion of protoplasm which is 

 thus capable of independent existence is known as a vege- 

 table cell, or protoplast. 



These free-swimming organisms are not protected by 

 any coating, but every part of their surface is in complete 

 contact with the water in which they live. This condition 



FIG. '2. SACCHABOMYCES CEBEVISLE. OB YEAST-PLANT, AS DEVELOPED 

 DUEING THE PROCESS OF FEBMENTATION. X 300. 



a, 6, c, d, successive stages of cell-multiplication. 



is, however, exceptional. Usually the protoplast is encased 

 in a colourless homogeneous membrane of extreme tenuity 

 which is known as its cell-wall. Examples of unicellular 

 organisms of this kind are found in great numbers among 

 the fungi, the Yeasts (fig. 2) and the Bacteria (fig. 3) being 

 exceptionally numerous. Such plants may be motile or non- 

 motile, a few of the bacteria being furnished with thread- 

 like appendages, known as cilia or flagella, which are 

 similar in most respects to those of the zoospores already 

 mentioned. These plants show a little more differentiation 

 than the others, the protoplasm being clothed by a kind 

 of exoskeleton, the cell-wall, which is at once supporting 

 and protective. 



More complex organisms consist of two or more proto- 

 plasts united together in various ways. The number of 



