334 PHYSIOLOGY 
Quantity exuded. In a few plants, especially in aroids, guttation under favorable 
conditions is so rapid that water drips from leaf tips or is even ejected. Thus a 
vigorous leaf of Colocasia has yielded 1008 cc. of water in 9 days, the water drop- 
ping at the rate of 85-100 drops per minute at times. C. nymphaeoides has been 
observed to eject a stream of minute droplets (at a rate of 195 per minute, so that it 
seemed almost a continuous jet of water) to a height of about i cm. 
Advantage? Seeing the structural features which permit guttation, 
one naturally asks, Is it advantageous? To that question no certain 
answer can be given. It is assumed that the free escape of water at 
these points prevents its escape elsewhere, and therefore prevents the 
infiltration of the aerating system with water, which would greatly retard 
the entry of gases and so the manufacture of food. But there are so 
many plants which lack the arrangements for guttation that one must 
doubt if this answer be adequate. 
Bleeding. Bleeding may be observed when vines are pruned rather 
late, or in many cases when a potted plant is decapitated. It must be 
distinguished from exudation due to heating the water and especially 
the gases contained in the woody parts of a plant, which has the same 
general effect. Thus, when a green stick is put on the fire, the scanty sap 
presently boils out of the ends; for the expansion of the gases and of 
the water, and later the steam generated by the fire, drive it out forcibly. 
Or if on a cold day in winter, one bring into a warm room a branch of 
a shrub or tree, water will soon ooze out at the cut surface. Here the 
gases in the wood are warmed (for though fuller of water in winter than 
at other times, the wood is never free from gases, else no green wood 
would float) ; they expand, and press upon the free water, forcing it out 
at the nearest opening. True bleeding, however, is restricted to live 
plants and is quite independent of any gas pressure due to heat. 
Industrial applications. Collecting maple sap for sugar or sirup making is 
partly an industrial application of bleeding. The work is often begun when only 
the heating of the twigs on a warm sunny day is active in forcing out the water 
through the wound made in the trunk; but a great part of the later exudation is 
dependent on other causes and must be accounted to this extent as true bleeding. 
Another commercial application of bleeding is found in the collection of the sap 
of various species of Agave in Mexico and Central America for the manufacture of 
fermented and distilled liquors. The process begins with cutting out the huge bud 
at the time when the plant, at the end of 5-15 years' growth, is about to send up the 
great flower stalk, 12-20 cm. in diameter and 6-10 m. high. Into the basin formed 
by removing the bud, the plant exudes several liters of water a day, for two months 
or more; this is collected daily, and after the addition of milk and fermentation is 
esteemed as a beverage, called pulque. Extensive plantations are devoted to rais- 
