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 1 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 devrted to rais- 



