LEAVES 



621 



uded from the ends of the principal veins, collecting as drops. The water 

 passes to the exterior through structures, known as water stomata, which 

 differ from ordinary air stomata in their position at the ends of veins, 

 in always remaining open, in the lack of the peculiar cutinization char- 

 acteristic of air stomata, and often by their large size (as in Tropaeolwn, 

 fig. 911). Underneath the aperture is an air cavity, below which is 

 a loose tissue, the epithem, made up of small 

 delicate cells without chlorophyll ; under- 

 neath this are diverging terminal tracheids, 

 representing the end of a conductive bundle 

 (fig. 912). The entire structure, consisting 

 of tracheids, epithem, and water stoma is 

 called a hydathode {i.e. water way). In 

 many plants, as in Primula and Fuchsia, the 

 hydathodes occur at the 

 tips of bluntish leaf teeth 



(%9i3)- 



The distribution and 



significance of hyda- 

 thodes. — Not much is 



known concerning the 



general distribution of 



hydathodes by habitats 



or regions; commonly 



they are regarded as most 



characteristic of the 



humid tropics, although 



present in many herbs 



of low grounds and humid 

 woods. The amount of excreted water may be very large; the tips of 

 Colocasia leaves have been known to drip water at the rate of 190 drops 

 per minute, and the leaves of Conocephalus are said to lose in one night 

 an amount equal to a fourth of their weight. The substance excreted is 

 nearly pure water, o.i per cent or less representing the proportion of 

 salts ; closely related to hydathodes are the chalk glands of Saxifraga 

 and the salt glands of Tamarix, which excrete, respectively, large quan- 

 tities of calcium carbonate and sodium chlorid. The prevalent theory 

 as to the advantage of hydathodes is that they are a means of escape for 

 a surplus of water in plants with high turgor pressure ("root pressure"), 



/ me 



Fig. 912. — A radial 

 longitudinal section through 

 a leaf tooth of the Chinese 

 primrose (Primula sinensis), 

 showing a hydathode with 

 divergent tracheids (/) termi- 

 nating the conductive bundle, 

 above which is the colorless 

 epithem (d), the stomatal 

 cavity (c), and one of the 

 guard cells (x) of the water 

 stoma at the tip of the tooth ; 

 i, intercellular air spaces in 

 the epithem; e, epidermis; m, 

 chlorenchyma ; highly magni- 

 fied. — From Habeelandt. 



Fig. 913. — A por- 

 tion of the leaf margin 

 of Fuchsia, showing 

 blunt teeth (t), each 

 of which represents a 

 hydathode at a vein 

 terminus. 



