2IO 



SECRETION AND EXCRETION 



Fig. 1 20. — Hydatode from the 

 leaf of Phaseolus multiflorus. 

 (After Haberlandt.) 



In another class of hydatodes bundles of tracheids from the 

 terminations of the vascular bundles supply water to water- 

 excreting parenchyma cells or themselves excrete water directly 



into intercellular spaces. Hydatodes of 

 this sort are not uncommon in ferns 

 where they occur chiefly along the leaf 

 margins. This type reaches its 

 highest development where water- 

 stomata are present in the epidermis 

 through which water excreted into the 

 intercellular spaces is expelled. This 

 is illustrated by Primula sinensis where 



the hydatodes occur in the teeth of the leaf margins (Fig. 121). 

 Physiological experiments show that many hydatodes excrete 



water through the activity of their living cells, since excretion 



stops after they have been killed by 



brushing over with a solution of cor- 

 rosive sublimate, and they cannot then 



be forced to excrete water even when 



the pressure in the tracheal tubes is 



made very great by connection with a 



U-tube with its long arm filled with 



mercury. The majority of hydatodes 



equipped with stomata will still excrete 



water, however, after they have been 



poisoned, and by artificially increasing 



the pressure in the tracheal system ex- 

 cretion of water is hastened. 



The amount of water excreted by 



hydatodes is often remarkably large. 



A young leaf of Colocasia nymphsefolia 



gave out 48 to 97 cubic centimeters 



of water in one night, and a mature 



leaf of C. antiquorum excreted on the average 9 to 12 cubic 



centimeters nightly. 

 The use of hydatodes seems to be that when transpiration 



Fig. 121. — Radial longitu- 

 dinal section through a hyda- 

 tode from the leaf margin of 

 Primula sinensis, i, upper, and 

 ;, lower epidermis; h, palisade 

 cell; e, thin-walled parenchyma, 

 called epithem; g, intercellular 

 space; /, guard cell of a water 

 stoma; k, tracheal elements 

 (After Haberlandt.) 



