266 HIGH SCHOOL ZOOLOGY. 



Plants as well as animals are provided with organs of these 

 three categories, but important differences exist as to the 

 nature of the processes discharged by them. The food of plants 

 is drawn from the inorganic world, and consists of salts 

 absorbed in solution from the soil, and of the carbonic acid in 

 the atmosphere. This is decomposed by the combined action of 

 the chlorophyll within the green plant-cells and light — part of 

 the energy of the sun's rays being intercepted by the colouring 

 matter — with the result that carbon is secured for forming simple 

 eai'bohydx'ates like glucose, which are afterwards elaborated 

 within the plant-body, while oxygen is disengaged far in excess 

 of that required for the ordinary pi-ocesses of oxidation. 



Thus, from simple compounds, in which little energy is lock- 

 ed np, more complex ones are formed, whicli are highly energised, 

 and which thus» serve for fuel and food to the animal world. 

 Animals are, in fact, dependent either directly or indirectly for 

 their food on the vegetable world, and, as far as we know, not 

 even the simplest among them (ajjart perhaps from such colour- 

 ed forms as are referred to in IX, 19) can derive carbon from 

 the inorganic world. 



(bj Even more striking differences between plants and 

 animals are to be met with in those organs which I'elate the in- 

 dividual to its environment ; in fact, the oi'gans of locomotion 

 and sensation are generally described as the specially " animal" 

 organs. Not that vegetable protoplasm is destitute of con- 

 tractility and irritability, but the further elaboration of these 

 essential properties of pi'otoplasm is chai'acteristic of animals, to 

 the exclusion of plants. Nor is it meant that the plant is less 

 adapted to its surroundings on account of the absence of special 

 organs of relation ; on the contraiy, we shall see that the plant 

 is just as plastic as the animal in its adaptability to its environ- 

 ment. The difference between them in this respect is no doubt 

 to be largely attributed to differences referred to in last j^ara- 



