4 A TEXTBOOK OF THEORETICAL BOTANY 



All living organisms exhibit irritability, that is they respond to external 

 influences which disturb the metabolic balance. This response generally 

 takes the form of movement. The phenomenon of movement varies 

 greatly in degree, and though very marked in animals is not so readily 

 observed in plants owing to their slower reaction to stimuli. 



The power of reproduction is common to all living organisms, that is 

 to say, the formation of new individuals which are similar to those already 

 existing. In its simplest form reproduction is merely an outcome of the 

 power of growth, involving the separation of part of the body. In this way 

 the race of organisms is perpetuated. 



We shall see that all plants exhibit phenomena which are characteristic of 

 life, but that the ways in which these are manifested are often very different. 



The protoplasm of even the simplest organism is divided into different 

 parts, a phenomenon spoken of as differentiation ; hand in hand with this 

 goes the allotting of special duties to the several parts so differentiated ; 

 this is called division of physiological labour. Probably there is no 

 organism without any differentiation at all and with a body consisting of a 

 homogeneous mass of protoplasm, though we shall have occasion to describe 

 some in which there is apparently very little. At the other end of the scale 

 there are organisms, all the parts of whose bodies are differentiated for 

 various functions. 



It is this progress towards specialization which forms the basic principle 

 of modern Biology. We can conceive that life first originated in the far- 

 distant past, by some means unknown, as a minute undifferentiated mass 

 of protoplasm. From this start there has been a gradual increase in com- 

 plexity in the descendants, resulting in highly specialized organisms. This 

 idea of the transition from the less specialized to the more specialized is called 

 evolution. The whole world has been populated by evolution with animals 

 and plants which are specialized for their particular surroundings — -that is to 

 say, they are adapted to their environment. 



Differences between Plants and Animals 



At the outset of evolution two main lines diverged, resulting in the two 

 great groups of the living world : the Plant Kingdom and the Animal 

 Kingdom. Different methods of nutrition caused these kingdoms to evolve 

 along completely separate paths, resulting in such radically contrasting 

 types as a tree and the monkey which climbs in it. 



Since animals require proteins, and since plants alone have the power 

 of forming them from inorganic substances, it necessarily follows that all 

 animals are dependent upon plants for their food supply. 



The other important difference between animals and plants is the result 

 of their different modes of nutrition. Plants are able to obtain the necessaries 

 of life from the air and the soil, which they can find almost anywhere, and 

 as a consequence they have no need of locomotion. 



Animals, on the other hand, in their search for ready-made proteins, 



