44 PHYSIOLOGY. 



By an examination of our leaf section we see that the intercellulai spaces 

 are all connected, and that the stomata, where they occur, open also into 

 intercellular spaces. There is here an opportunity for the water vapoi 

 in the intercellular spaces to escape when the stomata are open, 



84. Action of the stomata. The guard cells serve an important func- 

 tion in regulating transpiration. During normal transpiration the guard 

 cells are turgid and their peculiar form then causes them to arch away 

 from each other, allowing the escape of water vapor. When the air becomes 

 too dry transpiration is in excess of absorption by the roots. The guard 

 cells lose some of their v/ater, and collapse so that their inner faces meet 

 in a straight line and close the stoma. Thus the rapid transpiration is 

 checked. Some evaporation of water vapor, however, takes place through 

 the epidermal cells, and if the air remains too dry, the leaves eventually 

 become flaccid and droop. During the day the effect of sunlight is to 

 increase certain sugars or salts in the guard cells so that they readily be- 

 come turgid and open the stomates, but at night the cell-sap is less con- 

 centrated and the stomates are usually closed. Light therefore favors 

 transpiration, while in darkness transpiration is checked. 



85. Compare transpiration from the two surfaces of the leaf. This can 

 be done by using the cobalt chloride paper. This paper can be kept from 

 year to year and used repeatedly. It is thus a very simple matter to make 

 these experiments. Provide two pieces of glass (discarded glass nega- 

 tives, cleaned, are excellent), two pieces of cobalt chloride paper, and some 

 geranium leaves entirely free from surface water. Dry the paper until it is 

 blue. Place one piece of the paper on a glass plate; place the geranium 

 leaf with the under side on the paper. On the upper side of the leaf now 

 place the other cobalt paper, and next the second piece of glass. On the 

 pile place a light weight to keep the parts well in contact. In fifteen or 

 twenty minutes open and examine. The paper next the under side of the 

 geranium leaf is red where it lies under the leaf. The paper on the upper 

 side is only slightly reddened. The greater loss of water, then, is through 

 the under side of the geranium leaf. This is true of a great many leaves, 

 but it is not true of all. 



86. Negative pressure. This is not only indicated by the drooping of 

 the leaves, but may be determined in another way. If the shoot of such a 

 plant be cut underneath mercury, or underneath a strong solution of eosin, 

 it will be found that some of the mercury or eosin, as the case may be, will 

 be forcibly drawn up into the stem toward the roots. This is seen on 

 quickly splitting the cut end of the stem. When plants in the open cannot 

 be obtained in this condition, Oiie may take a plant like a balsam plant 

 from the greenhouse, or some other potted plant, knock it out of the pot, 

 free the roots from the soil and allow to partly wilt. The stem may then 

 be held under the eosin solution and cut. 



