184 A TEXTBOOK OF GENERAL BOTANY 
Bleeding is largely responsible for the exudation of maple sap 
or the sweet juices from palm inflorescences (Figs. 137, 158) 
that have been cut and that yield sugar or, after fermentation, 
‘produce alcoholic beverages or vinegar. The Mexican alcoholic 
beverage known as pulque has a similar origin, as it is the fer- 
mented juice obtained from agave plants from which the buds 
have been removed. | 
Absorption of minerals. The water around the soil particles 
eontains mineral matter which is dissolved from the soil. This ~ 
water is in contact with that in the cell wall of the root hair, 
and so is continuous with it, while the water in the cell wall is 
in turn continuous with that in the interior of the hair. The 
mineral matter dissolved in the soil water tends to diffuse into 
the water in the cell wall, and through that into the interior 
of the cell. In this way mineral matter passes from the soil 
into the plant. While in the root hairs the plasma membrane, 
or outer layer of protoplasm, is impermeable to sugar and many 
other substances within the cell, it is permeable to many of the 
simple inorganic compounds found in the soil. The diffusion 
of the mineral matter through the plasma membrane is inde- | 
pendent of the movement of the water. 
By the use of a piece of parchment paper separating a solu- 
tion of sugar from pure water it is very easy to demonstrate that 
the movement of a solvent and the movement of the solute 
through a membrane may be independent of each other. The 
volume of the sugar solution will increase, showing that it has 
absorbed water, while at the same time some of the sugar will 
pass in the opposite direction into the pure water. The movement 
of water in one direction and of a dissolved substance in the 
opposite direction, through the same membrane at the same time, 
can be strikingly shown by. the use of the thistle-tube apparatus 
shown in Fig. 32. The bulb of the thistle tube is filled with con- 
centrated sugar solution, a piece of parchment paper tied over 
the mouth of the bulb, the tube inverted, and the bulb immersed 
in distilled water. Water is drawn into the sugar solution, as 
is shown by the fact that the latter rises in the thistle tube. If 
