332 PHYSIOLOGY 
2. EXUDATION OF WATER 
Forms of exudation. Besides the vapor which constantly exhales 
from plants, liquid water exudes from certain regions intermittently. 
The places whence it issues are, first, certain specially permeable areas 
of the permeable regions in an uninjured plant ; second, the conducting 
tissue when opened by some wound. Guttation is the escape of water 
in drops from uninjured plants. It occurs especially in leaves in the 
vicinity of the tips of main veins, where there are stomata, often enlarged, 
called water pores, through which water exudes. Bleeding is the oozing 
of water from the water-conducting tissues when ruptured. It is espe- 
cially notable in the spring, before the foliage is fully developed. Secre- 
tion consists in the exudation of water and solutes from certain special- 
ized cells, constituting a gland, and found on various parts of plants, but 
especially on foliage and flower leaves. All these processes are essen- 
tially similar, with minor differences. 
Guttation. Guttation may be readily observed by inverting a glass 
jar over grass seedlings growing in well-watered soil and thus checking 
the evaporation. In a short time a water drop accumulates at the tip 
of the blade and enlarges until it runs down or falls off. Leaves of vigor- 
ous plants of many species (e.g. aroids, fuchsia, cabbage, nasturtium) 
under like conditions show droplets of water at the tips, or at marginal 
teeth, or near the end of main ribs. 
Accessory structures. In all these cases an examination shows es- 
sentially the same features: (a) a rift in the epidermis, or one or more 
water pores, over (6) a rather large chamber, which is bounded by (c) 
more or less specialized colorless parenchyma cells (epithem'), and near 
by (d) the tracheids at the end of a vein. The rift in the epidermis may 
be due (as in grasses) to growth and consequent stretching and rupture. 
The water pore is simply a deformed stomatal apparatus whose dilated slit 
is always wide open because the distorted guard cells are no longer motile. 
When the water pore is single, it is usually greatly enlarged and deformed ; 
when there are a number together, each is more nearly like an ordinary 
wide-open stoma. The cells lining the substomatal chamber differ from 
the mesophyll cells chiefly in lacking chloroplasts. They resemble the 
sheath of colorless cells, the so-called transfusion tissue, that adjoins the 
tracheids, which form the endings of the water-conducting bundles of 
the leaves. In some cases this epithem seems to be a water-secreting 
