64 Plants and their Ways in South Africa 



the centre of the cell's life and activity. It appears in its rest- 

 ing state as a denser shining portion of the protoplasm, spheri- 

 cal or lens-shaped. Volumes have been written on its structure 

 and behaviour, the rhythmic dance of its parts in the activity 

 of division which initiates cell division, and the preparation it 

 undergoes when about to fuse with another nucleus in the re- 

 productive process. A larger book must be consulted for these 

 changes. 



(iii) The plastids are special little organs embedded with 

 the nucleus in the cytoplasm. Like the nucleus, these are 

 formed from pre-existing plastids. They have their special 

 work to do and are of three kinds : — 



{a) Chloroplastids, bodies which contain the green 

 colouring matter by virtue of which the plant is enabled to ap- 

 propriate the energy of the sun's rays for photosynthesis (see 

 Chapter XII.). In brown and red seaweeds other colouring 

 matter masks the green. By dipping the plants in boiling 

 water these colours will be extracted and the green colour will 

 then become visible. By dipping again in boiling alcohol the 

 green colour will be extracted. 



{b) Leucoplastids, which are bodies similar to chloro- 

 plastids but are colourless. They are often found in parts 

 concealed from light, as in potato tubers. When these parts 

 are exposed they may develop chlorophyll. 



[c] Chromoplastids contain a substance which gives 

 colour to most yellow and red flowers and to carrots. When 

 fruits ripen or leaves take on their autumn tints of yellow, 

 chloroplastids change to chromoplastids. 



(2) Cell Sap. — A young cell is filled with protoplasm. 

 In older cells the protoplasm does not increase in proportion to 

 the cell wall. Several vacuoles and in time a large central 

 cavity are formed within it. These are filled with cell sap. 

 It consists of water in which are dissolved food material and 

 prepared foods. Blue and some red flowers owe their hues to 

 colouring substances dissolved in the sap. 



(3) Cell Walls. — Not all cells possess walls ; bright orange 

 masses may be found on pine needles in damp places. They 

 are a stage of a low class of plants known as the Myxomycetes. 



