18 GROWTH AND CHANGES. 



The color of plants resides in their cellular substance, beneath the 

 scarf skin, or epidermis, and depends on the peculiar functions of their 

 organs, their situation, heat, etc. Green leaves placed in the dark 

 become yellow, and then white. Young leaves grown in the dark, 

 turn from white to yellow, and then to green, on exposure to light. 

 Blossoms raised in the dark, are not materially changed by light. 

 Plants are lighter by combining with oxygen, and darker on parting 

 with it. Completely saturated with it, they become yellow, as with 

 the leaves in autumn ; but under other circumstances, when exposed, 

 they turn to green. The light of a lamp and that of the moon pro- 

 duces no sensible difference in effect. 



Secretions and excretions are likewise remarkable functions of plants. 

 All that is healthful and nutritive they secrete for their food and de- 

 velopment, and all that is baneful and unproductive they reject and 

 excrete through their roots. These, withdraw from the soil its various 

 qualities, which constitute their life, health, and the perfection of their 

 fruit ; combining and assimilating all that is essential for these pur- 

 poses and casting off all that is useless or poisonous ; yet that which 

 may be eminently useful, nevertheless, for other plants. 



The existence and growth of plants depend, as with animals, on the 

 reception and assimilation of food. A knowledge, therefore, of the 

 kind of nutriment they require is of great importance in vegetable 

 physiology, as well as in practical agriculture. A beautiful connec- 

 tion is thus seen between the organic and inorganic kingdoms. In- 

 organic matter affords food for plants, plants afford food for animals, 

 and both afford food for man. Men and animals require substances 

 that have life and organization ; but plants require inanimate and in- 

 organic matter. Both are apparent machines of greater or less com- 

 plexity, each depending on the other and acting to produce a deter- 

 minate end. 



The changes produced in plants by the assimilation of the various 

 substances of which they are composed, are the results of chemical 

 action and are traceable from the germ to the full-grown plant and 

 fruit. Water arid carbon are resolved into their constituent parts, 

 and these enter into new forms and combinations to constitute their 

 solid portions. The hydrogen of the water unites with the carbon, 

 received through the leaves from the air, to form oils, resins, sugar, 

 etc. The oxygen of the water combines with fluids to form acids, 

 etc. and is also given off from the leaves in the form of gas. 



The reproduction of plants is by evolution, which in process and 

 effect, is similar to that of animals. They are endowed with organs 

 which distinguish sexes and which are generally observable, but 

 which change after evolution. The polen or farina, the seminal prin- 

 ciple of plants, is contained in vessels called anthers. A part of this 

 penetrates the stigma, the head of the pistil, and is conveyed to the 



