44 PHYSIOLOGY. 



By an examination of our leaf section A^'e 5ee tliat the intercellulai spaces 

 are all connected, and that the stcimata, \vherc tlu-v occur, ojjcn also into 

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

 in the intercellular spaces to escape when the stoiuata are open 



84. Action of the stomata. — lire guard cells ser\'e 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. \\'hen the air fiecomes 

 too dry transpiration is in e.xcess of absorption b\' the roots. The guard 

 cells lose some of their water, 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, howe\Tr, takes place through 

 the epidermal cells, and if the air remains too dr\', the leaves eventuallv 

 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 readilv be- 

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

 centrated and the stomates are usually closed, flight 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 jiaijcr can be kept from 

 year to year and used rc|.>eate(lly. It is thus a \er\' simple matter to make 

 these experiments. Provide two pieces cif glass (dis( arded glass nega- 

 tives, cleaned, are e.xcellent), two pieces of cobalt chloride paper, and some 

 geranium lea\-es entirely free fr(.>m surface ^\"ater. ])r\- the pa])er until it is 

 blue. Place one piece of the paper on a glass ['late; place the geranium 

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

 place the other cobalt pa[)er, and next the seoral ]>iece of glass. On the 

 pile place a light weight to keep the jjarts 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 lo.ss 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, f)ut may be determined in another wav. If the shoot of such a 

 plant be cut \inderneath mercur\", or underm'ath a strong soluliop, of eosin, 

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

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

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

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

 frrjm the greenhouse, or some other potted plant, knock it out of the ]Hit, 

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

 be held under the eosin solution ami cul. 



