32 ^ INTRODUCTORY. PLANTS AND ANIMALS 



through the cell walls, so that all, or nearly all, the 

 protoplasm of the plant really forms a single connected 

 structure. 



The protoplasm of many of the cells of the bodies 

 of the higher plants disappears during the life of the 

 plant, so that only the dead cell walls remain, for in- 

 stance in the outer bark and the heartwood of tree 

 trunks, which are thus dead, though integral parts of 

 the living plant body, just as are the nails, for instance; 

 in the case of man. 



Green Colour of Plants. Chlorophyll. — The great 

 majority of plants, from the simplest algae to the 

 flowering plants, are green, or have green parts (fungi 

 and bacteria are an exception). It has already been 

 mentioned that this green colour is due to the presence 

 of a pigment called chlorophyll, whose presence, owing 

 to the light which it absorbs, enables the protoplasm 

 of the plant cell containing it to build up organic 

 substances out of simple inorganic substances which 

 the cell absorbs. The chlorophyll is contained in 

 definite protoplasmic bodies called chloroplasts, usually 

 spherical, oval or disc shaped, within the general 

 protoplasm of the cell. This process of the formation 

 of organic substances from inorganic by the activity 

 of the chloroplasts under the action of light is called 

 photosynthesis.^ It is upon photosynthesis that the con- 

 tinuance of life in the world, ultimately depends, because 

 it is the only natural process which makes new organic 

 substance on a sufficiently large scale. 



Protoplasm, as has been said, is the seat of all the 

 essential life processes. Before we can begin to analyse 

 the nature and relations of these it is essential to learn 

 something of the nature of protoplasm itself. Though 



■ " A putting together with the help of light," from Greek dcog, 



