Nutrition of Multicellular Plants - 251 



ing water from the surface of the thin-walled 

 chlorenchyma cells inside the leaf, and the 

 resulting vapor passes out of the leaf by way 

 of the stomatal openings. 



In most leaves the stomata occur chiefly or 

 entirely on the lower surface; and the num- 

 ber of stomata varies between 50 and 500 

 per square millimeter of the surface (Fig. 

 13-7). The average size at maximal opening 

 is about 6X 18 microns, so that about l /., () 

 of the leaf surface may be occupied by sto- 

 mata. The transmission of gases through 

 such a perforated membrane is almost as 

 rapid as in free diffusion. However, the size 

 of the stomata is regulated strictly by the 

 guard cells. When plenty of water comes up 

 to the leaf from the soil, the guard cells re- 

 main turgid; and when turgid, the guard 

 cells spring apart, opening the stomata (Fig. 

 13-18). But when the loss of water by trans- 

 piration exceeds the gain of water from the 

 soil, the guard cells wilt, and the wilted 

 guard cells change shape in such a fashion 

 that they block the stomatal openings. The 

 guard cells possess chloroplasts, which enable 

 them to regulate their own turgor according 

 to current conditions. By synthesizing sugar, 

 the cells may increase their turgor, or by 

 converting sugar into starch, the cells may 

 lose turgor. Accordingly the stomata tend to 



Fig. 13-18. The action of guard cells. Above, the 

 guard cells are turgid, leaving the stoma open; below, 

 the guard cells are wilted (i.e., less turgid), closing 

 the stoma. 



remain open in the daytime — provided the 

 water supply is adequate — but at night they 

 tend to be partially closed. 



The quantity of water transpired by an 

 average plant in sunlight is about 50 grams 

 per square meter of leaf surface per hour. 

 Thus a single corn plant puts forth more 

 than 50 gallons of water in the course of one 

 summer; or an acre of corn transpires about 

 300,000 gallons of water in the same time. 

 An average tree transpires more than 1500 

 gallons annually, and the total quantity of 

 water vaporized from the vegetation of a 

 forested region has a significant influence 

 upon the rainfall, humidity, and tempera- 

 ture of that region. 



As the sun beats down, the leaf absorbs 

 about 75 percent of the impinging light. 

 However, roughly only 3 percent of this 

 energy is utilized in photosynthesis. The rest 

 is transformed into heat — the heat that va- 

 porizes water and leads to transpiration. This 

 vaporization is most important, not only 

 because it dissipates the heat that otherwise 

 would kill the tissues of the leaf, but also 

 because it generates an osmotic force that 

 evacuates the ducts of the leaf and brings 

 about a further flow of sap upward from the 

 roots. 



Transpiration and the Flow of Sap. Transpira- 

 tion motivates the upward flow of sap by 

 altering osmotic conditions in the leaves. 

 When the chlorenchyma tissues lose water, 

 the cells become hypertonic to the sap in the 

 veins, which lie in close contact with the 

 chlorenchyma (Fig. 13-16). During transpira- 

 tion, consequently, water tends to be drawn 

 from the ducts into the chlorenchyma tissues. 

 Such a forceful evacuation of the ducts of the 

 leaf creates a lifting force that helps to ele- 

 vate the whole column of sap in its ascent 

 from the roots. At certain times, in fact, 

 when the solute content of the root sap is 

 low, transpirational lift represents the prin- 

 cipal, or even the only, force that maintains 

 an upward flow of sap. Transpirational lift 

 could not be effective, however, were it not 

 for the high cohesive strength of the aqueous 



