162 PART III. PHYSIOLOGY. [ 40 



a. The paratonic effect. 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 Jikewise 

 inhibits the spontaneous movements of motile leaves. 



b. 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 growth and of movement on being again exposed to light ; this 

 condition of motility induced by light is known as phototonus. 



c. The directive effect. The direction of the incident rays of 

 light effects 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 result 

 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 ; jor 

 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 ; chlorophyll 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 

 very small, though chlorophyll is developed, and heliotropic 

 curvature is well marked ; they soon die. 



40. 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 

 cell-sap, or of the cell-wall, of the constituent cells. The following 

 remarks apply especially to the higher terrestrial plants. 



a. The Tegumentary Tissue (pp. 106, 149), whether primary 



(epidermis) or secondary (periderm), has as its primary function 



: the mechanical protection of the underlying tissues : but it has 



the further functions of absorption and of preventing excessive loss 



of water by transpiration. 



