How the Plant Groics. 3 



leguminous plants, such as clover, alfalfa, and peas, take nitrogen 

 gas from the air and pass it on in combined form to the host plant, 

 thus indirectly supplying this important element. With these ex- 

 ceptions, the elements, as such, are never used in uncombined form 

 by plants, but serve them only when in chemical combination. 



2. Plant building. — Living matter is distinguished from non- 

 living matter by its power to grow, to repair its own waste, and to 

 reproduce itself. In plants the life principle is most in evidence in 

 the transparent, viscous protoplasmic masses found within the cells 

 of the green parts, principally the leaves ; and because of inherent 

 differences therein, each plant possesses an individuality and is able 

 to grow and reproduce itself after its own manner. 



The interior of the plant is everywhere bathed with juice or sap, 

 which is the great fluid medium for conveying the chemical com- 

 pounds, gathered by leaf and root, to the place where they are 

 formed into organized plant substances or building materials 

 proper, and, in turn, for transporting the materials thus elaborated 

 to all parts where needed. By means of this sap, the green-colored 

 protoplasm in the leaf cells is supplied with carbon dioxid taken 

 from the air by the leaves, and water, nitrates, and other soluble 

 mineral salts taken by the roots from the soil. The carbon dioxid, 

 salts, and water, commingling in the protoplasmic masses, are there 

 decomposed, and their atoms rearranged to form the various pri- 

 mary plant compounds. The first definite result of such union may 

 be some form of sugar or starch, with the excess oxygen given back 

 to the air as a free gas. It is thru the chlorophyll-containing pro- 

 toplasm of their leaves that plants are able, under the influence of 

 sunlight, to decompose carbon dioxid and water and to recast their 

 elements into such basal i)lant substances as sugar and starch. Sugar 

 and starch contain much energy which may be set free as heat 

 Avhen these substances are burned or otherwise broken up. Carbon 

 dioxid and water have little internal energy, and so on being de- 

 composed do not liberate heat. Energy must therefore be supplied 

 whenever sugar and starch are formed out of the elements con- 

 tained in these two energy-poor bodies. This energy comes from 

 the sun and is seized and used by the active life-holding protoplasm 

 in building carbon dioxid and water into energy-holding sugar and 

 starch. 



3. The carbohydrates.— Sugar and starch are the two great com- 

 mon elementary structural substances of plants. With their sec- 

 ondary products, the celluloses and pentosans, they constitute the 



