STRUCTURE AND FUNCTIONS 13 



are provided with special organs which vary in character 

 in the various groups. In the higher plants these 

 organs are much more complex in both structure and 

 function than the corresponding organs in the lower 

 plants. The organs are composed of tissues and the 

 tissues of cells or units. These cells or units are little 

 box-like structures, so small that they cannot be dis- 

 tinguished without the aid of a compound microscope. 

 They vary greatly in shape, size, and thickness of the 

 cell wall. In the dense parts of plants they are fitted 

 closely together, while in the softer parts they are much 

 less compact, usually larger and with thin walls. Not all 

 the cells of a plant are living, although the non-living 

 cells may contribute to the support of the whole plant. 

 The living cells contain a substance known as proto- 

 plasm, which is the real life substance of the plant. 

 This protoplasm, which is the same as the corresponding 

 material of the same name in animal cells, more nearly 

 resembles the white of an egg than any other substance 

 to which it can be compared. Many other substances 

 and bodies, such as colouring materials, starches, sugars, 

 fats, etc. are also found within the cells, but they and 

 the cell walls themselves are all dependent either directly 

 or indirectly upon the activities of the protoplasm. 



In some of the lowest plants these cells, which con- 

 stitute an individual, are all alike or very similar in both 

 structure and function. But in the higher plants, the 

 individuals are composed of groups of cells which may 

 differ widely in both structure and function. Such a 

 group of cells is known as a tissue, and these tissues are 

 so related as to form the organs of the plant. 



The vegetative organs of the plant are three in 

 number : (a) the leaves or foliage, which are primarily 

 for the purpose of utilizing the sunlight in the manu- 

 facture of starch, but which may also serve the purpose 

 of storage, holdfasts, etc. ; (b) the roots, which are 

 primarily to serve as holdfasts and to take water and 

 food from the soil, but which may also be modified for 

 various other purposes ; (c) the stems, which connect 



