1031 



impossibility of discovering the pores of this tissue by means of the 

 microscope. 



^^ Tlie cause of the ascent of the Crude Sap. — 20. Having now- 

 shown that the crude sap ascends in a porous tissue, universally 

 diffused through all parts of a plant, and occupying in greater or less 

 abundance the intervals between the cells, there will be but little 

 difficulty in comprehending the mode of its ascent and general 

 diflfusion. If it be admitted that the crude sap is of less density 

 than the fluid contained within the cells, and, as the former is derived 

 directly from the earth, whilst the latter is mixed with various soluble 

 substances elaborated within the cells, doubtless must be so, it will 

 then be apparent that the position of the crude sap, in a tissue 

 situated all around the cells, is the best possible one for favouring its 

 passage by endosmose through their walls into their interior, and for 

 causing the intercellular tissue in the immediate vicinity of these cells 

 thus to be exhausted of its fluid. 



" 21. The intercellular- tissue being porous and generally continu- 

 ous, must of necessity, if deprived of its fluid in any one part, attract, 

 in consequence of its capillarity, that which is contained in the tissue 

 of the surrounding parts, and thereby cause the crude sap to move 

 successively from one situation to another in a direction dependent 

 upon the position of that portion of the intercellular tissue which is 

 being most rapidly exhausted ; so that when the tissue situated be- 

 tween the cells in the leaves is rapidly being deprived of its crude 

 sap in consequence of the passage of this fluid by endosmose from 

 the exterior to the interior of the cells, this portion of tissue will at- 

 tract the water from that situated around the cells in the petioles of 

 these leaves, which, becoming more or less exhausted, will attract in 

 like manner the water from the intercellular tissue of the stem, and 

 thus the crude sap will be drawn up successively from one part of 

 the stem to another, until the intercellular tissue in the root, becom- 

 ing deprived of a part of its water, will re-fill itself by attracting the 

 water from the earth through the porous cuticle which covers the 

 radicles : or, if it be a branch vegetating with its extremity placed in 

 a solution of the bichloride of mercury, as in the experiment first re- 

 lated, the intercellular tissue at its cut extremity will attract the solu- 

 tion from the vessel in which it is placed." — p. 9. 



Besides its office of subserving the ascent of the sap, the intercel- 

 lular tissue is the means of its lateral diffusion. The cells of the me- 

 dullary rays are longest in their horizontal diameter, and are sur- 

 rounded by intercellular tissue. 



