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 (b) 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 



