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 
Fic, 2.—SaccHAROMYCES CEREVISIE, OR YEAST-PLANT, AS DEVELOPED 
DURING THE PROCESS OF FERMENTATION. x 800. 
a, b, ¢, 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. 8) 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 zoospoores 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 
