112 ELEMENTARY BOTANY 
chlorophyll (Gk. chloros= green ; phyllon=a leaf), because it is 
most abundant in leaves. Some Protococci are found without 
cell-walls ; the cell-wall is, therefore, not an essential part of 
their structure. | 
Protococcus is described as a unicellular organism, for it 
consists of a single cell or mass of living protoplasm. It 
takes in food from the water in which it lives, digests it, 
builds it up into its own substance, protoplasm. It breathes, 
that is, it takes in oxygen and gets rid of carbon dioxide. It 
grows and reproduces itself by splitting into two, thus giving 
rise to separate organisms. Since Protococcus consists only 
of protoplasm, these properties characteristic of life must be 
the properties of protoplasm. 
_ It is very difficult to say exactly wind protoplasm is ; it is 
impossible to ascertain its exact chemical com pasitied for 
when chemists apply different reagents to it the living proto- 
plasm is killed, so that although they can to some extent 
find out the composition of dead protoplasm, that of living 
protoplasm is still unknown. We do, however, know that it 
consists of certain elements—carbon, hydrogen, oxygen, 
nitrogen, sulphur, phosphorus, and some mineral salts, and 
we shall see (Chapter XVIII.) that these substances must 
enter into the food of plants in order that they may be able 
to build up their protoplasm. © 
We may now define a cell as a mass of living protoplasm, 
containing a nucleus which is the most ‘porn part of the 
cell. It must be remembered that a cell is a solid structure 
having length, breadth, and thickness; it is often very — 
minute, not ; 455 of an inch in diameter. Many of the 
lowest plants are made up of a number of cells, similar to 
each other, and each resembling in its chief features a Proto- 
coceus cell, each therefore capable of performing, indepen- 
dently of each other, the functions necessary to life. Such an 
organism is described as multicellular. 
- ‘Differentia. AS plants develop they become gradually 
ton. differentiated into: (1) A root portion, and 
(2) a shoot portion, each having its special work. The 
