674 PART IV. — THE PHYSIOLOGY OF PLANTS. 



1). The assimilation of carbon dioxide by the chloroplasfcids will 

 only take place in the presence of light of considerable intensity ; 

 it is especially a function of the rays of low refrangibility, as will 

 be subsequently explained. This is also true of the first steps in 

 the assimilation of mineral nitrogenous food (nitrates). 



The most conspicuous mechanical effects, exhibited by plants of 

 all kinds are : — 



a. The paratonic eff'ect. All parts of plants grow more rapidly 

 in feeble than in strong light, as is shown by the excessive length 

 attained by the shoots of plants grown in the dark ; hence, light 

 exercises a retarding influence on the rate of growth ; it likewise 

 inhibits the spontaneous movements of motile leaves. 



h. The phototonic effect. Dorsiventral leaves, when growing, 

 generally cease to grow, and when motile, lose the power of move- 

 ment, if long kept in darkness ; but they soon regain the power 

 of movement on being again exposed to light; this condition of 

 motility induced by light is known as phototonus. 



c. The directive eff'ect. The direction of the incident rays of 

 light aifects the position of growing and other motile members : 

 these phenomena are designated by the general term heliotropism. 



The various influences of light are well illustrated by plants 

 grown in darkness, or etiolated plants. For instance, an etiolated 

 potato-shoot has a stem with excessively long internodes, a I'esult 

 of the absence of the paratonic effect of light ; very small leaves, 

 in consequence of the absence of the phototonic effect ; no chloro- 

 phyll, in consequence of the absence of the chemical action of 

 light. Etiolation can, however, be induced, not only in plants 

 which normally possess chlorophyll, but in others as well ; for 

 instance, Fungi grown in darkness exhibit the characteristic 

 excessive elongation. Again, plants grown in light of low re- 

 frangibility (yellow or red) show the elongation characteristic of 

 etiolation ; chloroph^^ll is formed, and the leaves are fairly well 

 developed, but there is no heliotropic curvature : grown in light 

 of high refrangibility (blue), the stem is stunted and the leaves 

 veiy small, though chlorophyll is developed, and heliotropic 

 curvature is well marked ; they soon die. 



§ 4. The Functions of the Tissues. In dealing with this 

 subject, it is important to distinguish between the vital and the 

 physical functions ; to distinguish, that is, the functions which 

 depend upon the activity of the living protoplasm, from those 

 which depend upon some chemical or mechanical property of the 



