CAUSES OF THE MOTIOX OF JUICES. 385 



so that the direction of the movement of these organized 

 matters is reversed. In both years the motion of water 

 is always the same, viz., from the soil upwards to the 

 leaves. * 



The summing up of the whole matter is that the nutri- 

 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 nutriment, and on the seat of growth or other action. 



3. 



THE CAUSES OF MOTION OF THE VEGETABLE JUICES. 



Porosity of Vegetable Tissues. Porosity is a 

 property of all the vegetable tissues and implies that the 

 molecules or smallest particles of matter composing the tis- 

 sues are 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 immersing it in water it swells, becomes more trans- 

 parent, and increases in weight. If the water be colored 

 by solution of indigo or cochineal, the fiber is visibly 



* The motion of water is always upwards, because the soil always 

 contains more water than the air. If a plant were so situated that its 



roots si ion Id steadily lack water while its foliage had an excess of this 

 liquid, it cannot be doubted that then the "sap" would pass down in 

 a regular How. In this case, nevertheless, the nutrient matters would 

 take their normal course. 



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