346 HOW CEOPS GEOW. 



ent substances in the plant are not absolutely confined to 

 any path, and may move in any direction. Tlie 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. 



§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 onlj- iti 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 



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