154 THE BIOLOGY OF A PLANT. 



The plant absorbs matter directly through the external surface ; 

 the animal partly through the external and partly through an 

 internal (alimentary) surface. The plant is able to absorb simple 

 chemical compounds from the air and earth, and kinetic energy 

 from sunlight ; the animal absorbs, for the most part, complex 

 chemical compounds and makes no nutritive use of the sun's 

 kinetic energy. By the aid of this energy the plant manufac- 

 tures starch from simple compounds, carbon dioxide, and water ; 

 the animal lacks this power. The plant can build up proteids 

 from the nitrogenous and other compounds of its food ; the animal 

 absolutely requires proteids in its food. And by manufacturing 

 proteids within its living substance, the plant is relieved of the 

 necessity of carrying on a process of digestion in order to render 

 them diffusible for entrance into the body. 



Still, great as these differences appear to be at first sight, 

 all of them, with a single exception, fade away upon closer ex- 

 amination. This exception is the power of making foods. 

 Plants and animals differ in form because their mode of life 

 differs ; but a wider study of biology reveals the existence of in- 

 numerable animals (corals, sponges, hydroids, etc.) which have 

 a close superficial resemblance to plants, and of many plants 

 which resemble animals, not only in form, but also in possessing 

 the power of active locomotion. The stomach of the worm, as 

 shown by its development, is really a part of the general outer 

 surface which is folded into the body ; and the animal, like the 

 plant, therefore, really absorbs its income over its whole surface 

 -oxygen through the general outer surface, other food- matters 

 through the infolded alimentary surface. 



In like manner it is easy to show that not one of the differ- 

 ences between the plant and animal is fundamentally impor- 

 tant save the power of making foods. The worm must have 

 complex ready-made food including proteid matter. So must 

 the fern ; but the fern is able to manufacture this complex food 

 out of very simple compounds. In terms of energy, the worm 

 requires ready-made food rich in potential energy ; the fern, 

 aided by the sun's energy, can manufacture food from matters 

 devoid of energy. 



Hence it appears, broadly speaking, that the fern by the aid 

 of solar energy is constructive, and stores up energy ; the earth- 



