268 TKANSFER OF WATER THROUGH THE PLANT. 



the tip of a leaf of Colocasia antiquoriim, and 22.6 grams of 

 liquid were collected in one night. From the j'oung leaves of 

 certain Aroids water is sometimes ejected in a fine jet to a 

 distance of a few inches.* In these and the previous cases the 

 liquid escapes through rifts. 



TRANSPIRATION. 



720. The evaporation of water from the surface of the younger 

 parts of plants exposed to the air makes, as has now been seen, 

 a continual draught upon the sources of -water-supply. But 

 •while evaporation from the free surface of water or from any 

 dead membrane ceases in an atmosphere satuiated with moisture, 

 there is some experimental evidence to sliow that, under certain 

 conditions of radiation, evaporation from tlie living plant may 

 continue to take place even when the atmosphere is completely 

 saturated. This difference between evaporation from a free sur- 

 face and that from a plant, although not full}- established, ren- 

 ders it advisable to emploj- for the latter plienomenon the terra 

 transpiration. This term is sometimes employed in Physics 

 with another signification ; but its prior use in Vegetable Tliysi- 

 ology should prevent any confusion. 



721. Stomata. Neither through the cutinized cell-walls of 

 the- epidermis, nor tlirougli the suberized cell-walls of cork, 

 can tianspi ration take place to any extent; ^ but at myriads 

 of points in the epidermis of leaves and young stems there are 

 minute orifices which permit tlio air outside the plant to come 

 into communication with the air within. It has been shown in 

 Part I. that these openings, the stomata, possess definite rela- 

 tions as regards position to the intercellular spaces below them, 



' Musset : Comptes Eendus, 1865. 



Muntingh(1672), according to a reference in Flora (1837, p. 717), noted the 

 projection of a small jet of water from the leaf of an Aroid, as from a fountain. 



'■^ "It is of the liighest significance that those plants wlduh are sulmierged, 

 or those pai-ts of |ilants which grow in the ground and therefore cannot Icse 

 ■water by transpiration, possess a cuticle which permits water and dissolved 

 matters to pass through with comparative facility ; while the parts growing 

 in the air have a cuticle of a different quality, through which water passes cn'y 

 ■with difficulty, and tlius tlicy are protected from too great a loss of water " 

 (Pfpffef: PHanzenphysiologie, i., 1881, p. 139). 



The amount of a(jueons vapor which can escape through cuticle is very 

 small. According to Bonssingault, .005 gram of water may evaporate in one 

 hoar from one square centimeter of the rind of an apple, ■while from the surface 

 of a peeled apple fifty-five times as much is lost (Agronomie, vi., 1878, Ji. 349). 



