LAWS OF VEGETATION. 75 



consequent condensation of the crude sap, a distillation by 

 which pure water is removed and the contents dissolved in it 

 concentrated. Its thin, expanded form, exposing a great sur- 

 face to the action of the dry air and the rays of the sun favors 

 this. A single sunflower has been known to throw off from its 

 leaves two pounds of water in twelve hours. The supply, 

 of course, comes from the spongioles. When the sun shines 

 the most intensely and the atmosphere is dryest, evaporation is 

 the most rapid, and if the supply from the root meet the demand 

 from the leaf, all the operations of life in the plant are most 

 intensely active. But if the supply from the root is not equal 

 to the evaporation from the leaf, then the cells, no longer dis- 

 tended by fluid, collapse, the leaf wilts, and if it remain long 

 in that condition, dies. To keep up the balance between the 

 waste and supply, a curious regulator is introduced, in effect, 

 though not in structure, like that placed upon the steam-engine 

 to keep the machinery driven at a uniform rate. This arrange- 

 ment consists of longitudinal openings (^stomata) through the 

 cuticle into the intercellular passages of the leaf. These open- 

 ings are bounded on either side by an elongated cell made fast 

 at each end ; as the cell fills and expands and the distance 

 between the two ends cannot be increased, it curves outward 

 and opens the orifice and the water exudes rapidly, but when 

 the cells are collapsed the curvature become less, the orifice is 

 closed and evaporation ceases except through the cell walls. 

 Very little evaporation takes place during the night, and you 

 have noticed that the corn leaves which rolled up during the 

 day expanded during the night by the continued activity of the 

 root. In transplanting, when the radicals are broken off so 

 that the proper supply of moisture is not taken up, the plant 

 droops, and you cover the leaves to protect them from the 

 evaporating force of the sun and air till new spongioles are 

 grown and the root is in a condition to do its duty. 



Another function of the leaf is to effect a chemical change in 

 the sap as it passes through it. The green color of the leaf is 

 due to a peculiar substance in its cells called chlorophyl, which 

 in connection with sunlight, produces a decomposition and 

 recomposition in the contents of the sap. I must be explicit 

 here, for the knowledge is interesting and important. Every body 

 knows that common charcoal exists in and can be obtained from 



