THE MATERIAL OUTGO OF PLANTS 335 



ing the agave or maguey, and pulque trains run into the large cities, as milk trains 

 do in this country. The fermented sap is also distilled to make various fiery alco- 

 holic drinks. 



Conditions. — The conditions under which bleeding occurs are like 

 those for guttation, a liberal water supply and limited transpiration; 

 that is, the conditions which permit maximum turgor. Even so, not all 

 plants bleed ; hence it cannot be at all necessary, nor can the causes be 

 universally active. 



Cause of exudation. — The cause of bleeding and guttation is to be 

 sought in the development of high turgor in certain cells (on account of 

 the osmotic pressure of the solutes in them to which the protoplast is 

 impermeable), which is made possible by adequate water supply. To 

 stop evaporation by making the air about the aerial parts very moist, or 

 by cutting away the aerial parts, or to have limited evaporation because 

 the foliage is not yet fully developed, are merely ways by which a water 

 supply, that might otherwise be barely enough to cover the evaporation, 

 is made ample; and this permits high turgor when other conditions are 

 met. When the turgor rises to a certain point in the active cells, it seems 

 that water is exuded. 



This may be mere filtration under pressure. But we may also conceive it to be 

 due to a sudden alteration of the permeability of the cytoplasm, wrought by the 

 very pressure itself. In that event, upon the relief of pressure when the outgo oc- 

 curs, there would be a gradual recovery of impermeability and consequently of 

 turgor to the maximum; than another automatic change of permeability, a conse- 

 quent outrush of water, and so on. 



This outflow naturally cannot be pure water : but on the theory of 

 filtration the water will contain at least the substances to which the pro- 

 toplast is permeable; and on the second hypothesis, any or all solutes 

 might be released, the sap as a whole escaping. In the water there are 

 often substances in small amount, regarding whose osmotic relations we 

 are ignorant, though the general assumption is that they could not pass 

 the cytoplasm without some special modification of its permeability. 

 When that is demonstrated, it will be necessary to adopt the second 

 hypothesis, which is also used to account for the presence of such sub- 

 stances in secretions (see p. 340). Until then it will suffice to assume 

 that they issue with the water because they are free to do so. 



Tissues concerned. — In the case of PUobolus and like plants, the tur- 

 gor which causes the escape of water evidently arises in the very cell or 



