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; then 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 Pilobolus and like plants, the tur- 
gor which causes the escape of water evidently arises in the very cell or 
