346 HOW CROPS GROW. 
ent substances in the plant are not absolutely confined to 
any path, and may move in any direction. The fact that 
they chiefly follow certain channels, and move in this or 
that direction, is plainly dependent upon the structure 
and arrangement of the tissues, on the sources of nutri- 
ment, and on the seat of growth or other action. 
8 3, 
THE CAUSES OF MOTION OF THE VEGETABLE JUICES. 
Porosity of Vegetable Tissues.—Porosity is an uni- 
versal property of massive bodies. The word porosity 
implies that the molecules or smallest particles of matter 
are always separated from each other by a certain space. 
In a multitude of cases bodies are visibly porous. In 
many more we can see no pores, even by the aid of the 
highest magnifying powers of the microscope; nevertheless 
the fact of porosity is a necessary inference from another 
fact which may be observed, viz., that of absorption. A 
fiber of linen; to the unassisted eye, has no pores. Under 
the microscope we find that it is a tubular cell, the bore 
being much less than the thickness of the walls. By im- 
mersing it in water it swells, becomes more transparent, 
and increases in weight. If the water be colored by solu- 
tion of indigo or cochineal, the fiber is visibly penetrated 
by the dye. It is therefore porous, not only in the sense 
of having an interior cavity which becomes visible by a 
high magnifying power, but likewise in having throughout 
its apparently imperforate substance innumerable channels 
in which liquids can freely pass. In like manner, all the 
vegetable tissues are more or less porous and penetrable 
to water. 
Imbibition of Liquids by Porous Bodies.—Not only do 
the tissues of the plant admit of the access of water into 
