170 



BIOLOGICAL LECTURES. 



transpiration, the surface of the plant may become for protec- 

 tive purposes hairy; the cuticular and epidermal layers may 

 be thickened; the interior air passages in the leaf which 

 communicate with the surface may become obliterated through 

 consolidation of the cellular structure; and, in certain cases, 

 the leaf may have added to it definite kinds of tissue for 

 storing water to be used in time of need. 



(2) That light readily causes various reactions in many of 

 our ordinary plants, every one knows. Watch the folding of the 



PLATE No. 4. PLATE No. 5. 



Clover leaves as daylight decreases, and you will see them go 

 into their sleeping position, while in the morning they open 

 their faces and present them to the sky. Look at the Wistaria 

 vine by the aid of a lamp, late at night, and you will see that 

 the little leaflets have dropped down and closed together. If 

 the morning is bright and warm, they will rise early and by 

 nine o'clock every young tender leaflet will point its tip directly 

 at the sun, in a direction parallel with both the light and heat 

 rays. As the heat of the sun increases toward noon and during 



