46 



PirVSIOLOGY. 



serious injury through lesions of the tissues. This is brrHif:^ht aliout liv the 

 cells at certain parts beconiini^ cliargc(l so full with water through the 

 activity of root pressure and lessened transpiration, assisted also probably 

 bv an accumulalion of certain acids in the cell-sap which lannot be got 

 rid of bv transpiration. I'nder these conditions some of the cells here 

 swell out, fornu"ng extensiv<' cushions, and the cell walls beeome so "weak- 

 ened that they burst. It is ]H)ssible to imitate the excess of root pressure 

 in the case of some plants by connecting the stems with a system of 



water jiressure, when vcrv r|uickly 

 the dro])S of water will begin to 

 exude from the margins of the 

 leaves. 



90. It should be stated that in 

 reality there is no difference between 

 transpiration and evaporation, if we 

 bear in mind that evaporation takes 

 place more slowlv from li\'ing plants 

 than from dead ones, or from an 

 equal surface of water. 



91. The esca[)e of water vapor is 

 not the only function of thestomata. 

 Tlie exchange of gases takes ])lace 

 through them as we shall later see. 

 A large number of experiments show 

 that normally the stomata are o]ien 

 Mhen the leaves are turgid. But 

 y-lii-n plants lose excessive quantities 

 (^f water on dr^' and hot davs, .so 

 that the leaves become Haccid, the 

 guard cells automatically close the 



Fig. 56- stonuita to check the escape of water 



Tlic roots are lifting more water into . Some water escaT)es through 



the ]tlant. than can be given orr m the lonn 1 1 ^ 



(it water vaiiur, sm it is pressed dut in tin' ei>idermis of manv plants, 

 drups. From '■ First Studies Plant Lite. " . , . ' ' 



though the culieulanzed mem- 

 brane of the epidermis largely jirevcnts evaporation. In arid regions 

 plants are usually provided with an epidermis of several la_\'ers of cells to 

 more securely ])revent evaporation there. In such cases the guard cells 

 are often prolei ted by being sunk dee]>l\" in the e|)idermal hna-r. 



92. Demonstration of stomates and intercellular air spaces. — A good 

 demonstration of the pre.-.eni e of stomates in lea\es, as well as tlw jiresence 

 and intercommunication of die iiileri elhilar spaces, can lie made bv blow- 

 ing into the cul "cw^A of the p(dioIe ol the leaf of a calla lily, the lamina being 



