THE VEGETABLE CELL 285 



certain cases two very small bodies lying usually side by side at 

 some point of its margin. These are called the centrospheres. 

 Each is spherical and hyaline, with a distinct outline, and 

 contains in its interior a central smaller body. This is known 

 as the centrosome. 



In rare oases a true cell may contain several nuclei. This 

 occurs in consequence of direct division or fragmentation of the 

 original nucleus. 



Besides these typical constituents the protoplasm of many 

 eeUs contains other differentiated structures, known as plastids. 

 These are special bodies which are charged with some definite 

 function and point to an early division of labour in the cell. 

 They are of a somewhat similar structure to the nucleus, being 

 formed of a fibrillar network supported by a ground substance. 

 Some of them are oolom-less, others impregnated with different 

 colours. The first are called leucoplasts, the others chloro- or 

 cJiromoplasis, according to their hue, the green ones or chloro- 

 phyll bodies being thus distinguished from all others {fig. 614). 



The plastids must be distinguished from other ceU-contents, 

 such as will be described later. They are independent bodies 

 charged with the performance of definite functions ; they always 

 lie in the substance of the protoplasm, and not in the vacuoles ; 

 they have a power of multiplication by division, quite apart from 

 the activity of the cell-protoplasm. Their shape and their 

 dimensions vary extremely in different cells, but they are gene- 

 rally spheroidal or discoid. There seems to be but little difference 

 between the chloroplast and the leucoplast, the latter being 

 formed in parts to which light has no access, the former occurring 

 in parts exposed to light. In the presence of light the leucoplasts 

 can develop a green colouring matter, becoming chloroplasts. 



The colouring matters of the different plastids can be ex- 

 tracted by various solvents, leaving a colourless basis. The 

 green colouring matter of the chloroplasts, which is known as 

 chlorophyll, can be dissolved out by alcohol, benzol, and other 

 fluids. The red or brown colour of the chromoplasts of the red or 

 brown seaweeds is soluble in cold distilled water. They are found 

 after removal of these pigments to contain chlorophyll as well. 



Chromoplasts of various colours give their tints to many of the 

 brilliant floral leaves, particularly those which are yellow or 

 scarlet. These are generally derived from chloroplasts, the 

 leaves when young being green. 



We have seen that, at some period of their life, many of the 

 lowliest plants consist only of a small ovoid mass of protoplasm 



