SOURCES OF MATTER AND ENERGY. 105 



production, and in all other forms of vital action, the plant uses 

 force and loses energy. In doing any kind of work, portions of 

 the organism are consumed and cast out as burnt or waste mate- 

 rials which mostly take the form of carbon dioxide and water ; 

 and these phenomena go on to a certain extent whether the plant 

 feeds or not. But it is evident that these losses of energy and of 

 matter must ultimately be made good by fresh supplies entering 

 the plant, and the income must equal the outgo if the fern is 

 merely to hold its own, and must exceed it if the fern is to grow. 



Thus it comes about that there is a more or less steady flow 

 of matter and of energy through the living organism, which is 

 itself a centre of activity, like a whirlpool (p. 2). The chemical 

 phenomena accompanying the flow of matter and energy through 

 the organism are those of nutrition in the widest sense. This 

 term is more often restricted especially to the phenomena accom- 

 panying the income, while those pertaining to the outgo are 

 regarded as belonging to excretion. The intermediate processes 

 directly connected with the life of protoplasm are put together 

 under the head of metabolism, though this word is sometimes 

 used in a broader sense to designate collectively all of the chem- 

 ical phenomena of protoplasm. 



The Income. The income of the fern is of two kinds, viz., 

 matter and energy. 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). 

 Energy may enter the plant to a small extent as the potential 

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

 sunlight absorbed in the leaves. The table on p. 106 shows the 

 precise nature and the more important sources of the income. 



Of these 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, a main though not the only cause of movement 

 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 thus may pass to every portion of the plant. 



Pteris owes its power of absorbing the energy of sunlight to 



