EXCRETION OF LIQUID WATER 



209 



drops, by the anatomy of the leaves where the drops occur, and 

 by physiological experiment, as will now be set forth. 



Droplets from the leaves of Indian corn on evaporation leave 

 behind .05 per cent, of solid residue, from leaves of Brassica 

 cretica i per cent., and so on for different plants; and on incinera- 

 tion 15 to 50 per cent, of this residue turns out to be ash. Real 

 dew being distilled water could not 

 be expected to give such results. 



Sections through a leaf where the 

 droplets occur are found under a 

 microscope to have specialized cells 

 or groups of cells having the appear- 

 ance of glands. These have been 

 named hydatodes. One of the simplest 

 of these is found on the upper and 

 under surfaces of the leaves of 

 Gonocaryum pyriforme, where each 

 hydatode consists of a single epi- 

 dermal cell differing from the rest in 

 several details, as shown in Fig. 119. ~^^ 

 A portion of the outer wall grows out 

 and forms a slender projection trav- 

 ersed longitudinally by a canal extend- 

 ing from the mucilaginous apex to the 

 cell cavity. The cell cavity is broad 

 and funnel-shaped in its upper part, 

 and again broadened below the neck 

 of the funnel. As is the rule with 



secreting cells, the cytoplasm is unusually dense and the nucleus 

 relatively large. In this instance water is absorbed by the 

 hydatode from surrounding epidermal and subepidermal cells 

 and excreted tl^irough the narrow canal in the exterior projection. 



On the under side of the leaves of Phaseolus multiflorus are 

 curved hairs, as shown in Fig. 120, with outer walls thin and 

 but little cutinized through which water filtrates and is excreted 

 at the surface. 



Fig. 119. — One-celled hydatode 

 of Gonocaryum pyriforme, seen 

 in cross section at A, and from 

 the surface in B. (After Haber- 

 landt.) 



14 



