234 BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 



550. The amount of water lost by transpiration varies very greatly 

 with the character of the plant and the conditions under which it is 

 placed. The early experimenter Hales, by weighing, determined the 

 loss from a potted Sunflower plant, three feet and a half high, to be 

 on the average one pound four ounces every twelve hours. From this 

 some idea may be formed of the very large weight of water transpired 

 by a full-grown tree on a warm day. It has been estimated that the 

 amount of aqueous vapor given off by an acre of Beech forest between 

 June 1 and December 1 is between 1000 and 1500 tons. 



551. The object of the transpiratory activity is the acquirement of 

 nutrient salts from the soil and their transportation to the leaves, 

 where they are left by the evaporation of the water. 



552. The rate of transpiration is regulated in part by the action of 

 the stomates. When the guard cells of a stomate are turgid the slit 

 between them stands w^ide open. If the guard cells become flaccid, 

 either through undue waiting of the leaf or from any other cause, the 

 stomatal opening becomes narrowed or closed. The guard cells are 

 sensitive to the influence of light; in bright sunshine the stomates 

 stand wider open than in diffused light, and they close on dark, stormy 

 days even in summer. Artificial darkness closes them — more quickly 

 in the afternoon than in the morning. At night the majority of plants 

 close their stomates, but not so as to prohibit all transpiration. The 

 stomatal cells are sensitive also to dryness. A draught of dry air 

 causes them to close, even though the leaves show no signs of 

 wilting. 



553. Aside from stomatic regulation, the rate of transpiration for 

 any given plant depends largely upon the external circumstances of 

 heat, light, dampness, or dryness of the atmosphere and supply of 

 water at the root. Heat furnishes the energy for all evaporation ; 

 consequently, rise of temperature in the leaf accelerates transpiration. 

 Light also has a stimulating effect. Dampness of the air around the 

 leaf, on the contrary, retards transpiration, just as it checks ordinary 

 evaporation. And of course dryness of the soil acts finally to reduce 

 the amount of transpiration. 



554. Assimilation of carbon. — Carbon is the most important of the 

 elements going to make up the solid parts of the plant body. How 

 great a proportion of the framework it forms is seen when wood is 

 subjected to great heat in the absence of air. Everything volatile 

 is then driven off; but the form remains, even the microscopic 

 details of structure being preserved by the carbon. Carbon con- 

 stitutes, by weight, about one-half of the dry substance of ordinary 

 plants. 



555. Carbon dioxide, the source of this important element, enters 

 the leaf through the stomates, passes along the intercellular spaces 

 of the spongy tissue, becomes dissolved in the water that saturates 



