34 2 Sutherland and Eastwood . — ■ The Physiological 
In very young leaves the tip of the hydathode projects a short dis- 
tance above the surface like a swollen glandular hair. This suggests a 
possible theory as to its ontogeny. The hairs so abundant in many land 
grasses would be useless to the plant when submerged. This is partly 
borne out by the fact that many hairs drop off before the leaves unroll 
and open. Then the hairs in Spartina correspond in position to the 
short cells between two long ones, just as the hydathodes do, and occa- 
sionally they have been observed occurring along the same lines as the 
latter in young plants. It is possible, therefore, that these excretory organs 
are really hairs modified to meet a new set of conditions. An extensive 
examination of the genus is necessary before a definite statement can be 
made with regard to the point. 
These hydathodes differ from any described forms, and are certainly 
unique in grasses. The nearest approach to them are the secretory 
cells, discovered by Sauvageau in aquatic Monocotyledons, and described 
by him in various papers on the structure of the leaves of these. In 
Cymadocea aequora , for example, they are distributed for the greater part 
along the margins of the leaves ; in others they may be scattered irregu- 
larly over the surface. These, however, are merely larger epidermal cells 
whose outer walls remain thin, and become distended and convex, while 
the inner portions penetrate slightly into the mesophyll. They resemble 
those described above in function, but lack their more definite structure. 
The Spartina type of hydathode may undoubtedly be regarded as 
a kind of safety-valve for getting rid easily and quickly of excess water 
and mineral salts, both of which, in abundance, are accessible, as a rule, 
to the plant. Thus fairly rapid loss of these is not dangerous to it, 
a fact which helps to explain the absence of any kind of epithem acting 
as a filtration-tissue, as in so many Dicotyledons. This want, common 
curiously enough to most Monocotyledons, facilitates rapid exudation. 
Their activity in this respect may be demonstrated very simply by cutting 
some plants and placing the cut stems in water under a bell-jar. In a few 
hours an immense number of drops may be seen on blade and sheath. The 
hydathodes forming the lines along the furrows function so quickly that 
tiny sparkling drops may be detected in less than an hour. These are 
much more active than those on the abaxial surface, owing doubtless to 
their greater proximity both to the large water-storing cells and the special 
assimilating tissue. 
Large quantities of salts, especially sodium chloride, are present in the 
excreted water. These may be detected by chemical methods in the drops 
given off, but a more striking demonstration of their presence can be seen in 
nature. While examining some plants, whose upper leaves and peduncles 
had been exposed continuously for some days of neap tides to a fairly dry 
atmosphere, our attention was directed to numerous, small, white, worm-like 
