146 THE BIOLOGY OF A PLANT. 



A little consideration will show that every function or action of living 

 things may be regarded as contributing to the same great end, viz., har- 

 mony with the environment ; and from this point of view life itself has 

 been defined as "the continuous adjustment of internal relations to ex- 

 ternal relations." * 



Nutrition. The fern does work. In pushing its stem 

 through the soil, in lifting its leaves into the air, in moving 

 food-matters from point to point, in building new tissue, in the 

 process of reproduction, and in all other forms of vital action, 

 the plant expends energy. Here, as in the animal, the imme- 

 diate source of energy is the living protoplasm, which, as it 

 lives, breaks down into simpler compounds. Hence the need of 

 an income to supply the power of doing work. 



The Income. The income of the fern, like that of the earth- 

 worm, is of two kinds, viz. , matter and energy, but unlike that 

 of the worm it is not chiefly an income of foods, but only of tfie 

 raw materials of food. Matter enters the plant in the liquid or 

 gaseous form by diffusion, both from the soil through the roots 

 (liquids), and from the atmosphere through the leaves (gases). 

 We have here the direct absorption into the body proper of food- 

 stuffs precisely as the earthworm takes in water and oxygen. 

 Energy enters the plant, to a small extent, as the potential energy 

 of food-stuffs, but comes in principally as the kinetic energy of 

 sunlight absorbed in the leaves. The table on p. 147 shows the 

 precise nature and the more important sources of the income. 



Of the substances, the solids (salts, etc.) must be dissolved 

 in water before they can be taken in. Water and dissolved salts 

 continually pass by diffusion from the soil into the roots, where 

 together they constitute the sap. The sap travels throughout 

 the whole plant, the main though not the only cause of move- 

 ment being the constant transpiration (evaporation) of watery 

 vapor from the leaves, especially through the stomata. The 

 gaseous matters (carbon dioxide, oxygen, nitrogen) enter the 

 plant mainly by diffusion from the atmosphere, are dissolved by 

 the sap in the leaves and elsewhere, and thus may pass to every 

 portion of the plant. 



The Manufacture of Foods especially Starch. Pteris owes 

 its power of absorbing the energy of sunlight to the chlorophyll- 



* Spencer, Principles of Biology, vol. i. p. 80. N. Y., Appleton, 1881. 



