FUNCTIONS OF HYDATHODES 501 



undoubtedly interfere with the nutritive metabolism of the leaves, by 

 retarding the gaseous interchange attendant upon the photosynthetic 

 process. The presence of hydathodes also ensures that, even when trans- 

 piration is entirely suppressed, a sufficient amount of water continues to 

 flow through the plant, carrying with it mineral salts which are retained 

 by the cells of the mesophyll. From this point of view hydathodes 

 must be regarded as organs of great importance in relation to nutritive 

 metabolism, especially in the case of plants inhabiting humid tropical 

 regions. The same argument applies to the " apical pores " of water- 

 plants, which, as Sauvageau, Minden and others have shown, likewise 

 keep up a flow of water throughout the plant-body. This " hydathode- 

 current " [or " secretion-current " | is, however, always feeble in com- 

 parison with the " transpiration-current " of land-plants, as indeed may 

 at once be inferred from the prevalent reduction of the water-conducting 

 system among aquatic plants. 



In the genus Zathraea, the activity of the hydathodes in the leaf- 

 cavities doubtless serves to draw large quantities of sugar-containing sap 

 from the host-roots into the parasite, and particularly into the scale-leaves 

 themselves. In this way the parasite can absorb a great amount of 

 plastic material in a short space of time ; as a matter of fact, the scale- 

 leaves do fill up rapidly with starch in springtime. 



In certain cases hydathodes are adapted for very special purposes, 

 and they may even become transformed into organs which perform 

 functions other than the secretion of water. The first observations on 

 such modified hydathodes were made by Treub upon the flowers of 

 Sjmthodea campanulata (Bignoniaceae). The calyx of this plant forms 

 a closed sac, which bears numerous trichome-hydathodes upon its inner 

 surface ; these secrete a large quantity of liquid, within which the 

 corolla and essential organs of the flower can develop without risk of 

 desiccation. Koorders has since shown that such water-calyces are 

 very widely distributed among tropical plants ; they occur, for example, 

 in several other Bignoniaceae, and also among the Solanaceae (Jochroma 

 macrocalyx, Nicandra physaloides), Yekbenaceae (Clerodendron), Scro- 

 phulariaceae, etc. The " post-floral " water-calyces discovered by 

 Svedelius in certain Convolvulaceae probably serve to protect the 

 developing fruit against premature desiccation. 245 



By a complete change of function hydathodes become converted 

 into digestive glands or into nectaries. 



