234 i;i:iEF orTLixE of vfaietable physiology 



550. The amount of water lo^t by transpiration varies very greatly 

 with tlie character of tlie 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 htrge weight of water tran.spired 

 by a fuU-growu tree on a warm day. It has been estimated that the 

 aniuuiit of ;u]ueous 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 acquii-ement 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 hy 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 wilting of the leaf or from any other cause, the 

 stomatal opening becomes nanowed or closed. The guard cells are 

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

 stand wider open than in diffused light, and they close on dark, stoi'my 

 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. 'J'he 

 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 f(.ir 

 any given plant depends largely upon the axternal circumstances of 

 Iii'at, 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 ft 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 

 elenu-'nts going to make up the soliil parts of the pilant bodj'. How 

 gi'i-at a ]irn]>ortiou of the frainewoi'k it forms is seen when wood is 

 sulijectcd to great heat in tlie absence of air. Everything volatile 

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

 details of structure lieing preserved by the carbon of the charcoal. 

 Carbon constitutes, by weight, aliout one-half of the dry substance of 

 ordiuary plants. 



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

 the leaf through the stomates, jiasses along the intercellular spaces 

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



