210 



SECRETION AND EXCRETION 



FIG. 1 20. Hydatode from the 



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 

 leaf of FhaseDius muitiflorus. intercellular spaces is expelled. This 



(After Haberlandt.) 



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 Primula sinensis ' *' upper ' and 



FIG. 121. Radial longitu- 

 dinal section through a hyda- 

 tode from the leaf margin of 



j, lower epidermis; h, palisade 

 cell; e, thin-walled parenchyma, 



. called epithem; g, intercellular 



A yOUng leaf Of ColocaSia nymphaelolia space; /, guard cell of a water 



elements 



hydatodes is often remarkably large. 



gave out 48 to 07 cubic centimeters f ma =*' 



(After Haberlandt.) 



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 



