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tissues of the plant, by diffusion ; their distribution, like that of 

 the sugar solution, being regulated by the principle of demand 

 and supply. 



If we cut through and remove a ring of bark for the breadth 

 of one inch, down to the very surface of the wood, taking care, 

 also, to remove the cambium zone, from the stem of a willow, for 

 instance, since this forms a good subject for operating upon, we 

 shall find that the part of the stem below the ring is materially 

 injured, though the upper part is not so. If we leave it for some 

 time the upper part thickens, produces rootlets and buds, and also 

 branches vigorously ; while the part below the ring does none 

 of these. The upper part remains in connection with the leaves ; 

 but the removal of the ring of bark has interrupted the passage 

 of the substances formed in the leaves, both non-nitrogenous 

 and, as we shall see presently, nitrogenous also, which the lower 

 portion requires for its growth ; so that this part is deprived of 

 its nourishment, and, in consequence, it remains dwarfed and 

 arrested, while the rudiments of buds which were previously 

 present upon it remain undeveloped. By this experiment we 

 have shown that the nutritive substances from the leaves pass 

 by the elements contained in the outer layers of the stem, ex- 

 ternal to the wood, which we have removed. Ringing fruit 

 trees in this way causes a swelling of the tissues, and a tem- 

 porary increase in the production of fruit above the wound, 

 since there is less growing surface to be supplied. According 

 to Sachs, if we cut a transverse section of a herbaceous stem, 

 such as the cucumber, dry the cut surface and place neutral 

 test paper, previously moistened with water, upon it, we find 

 that certain parts of the test paper become red and certain 

 others blue ; — those parts which are colored red, indicating an 

 acid fluid (containing sugar, oil, and vegetable acids formed 

 from starch in various ways), correspond to the position of the 

 parenchyma cells of the plant, viz., the cambium, the medullary 

 rays, and the parenchyma of the bast layer ; while those which 

 are colored blue, and indicate an alkaline fluid (containing, 

 according to him, substances of an albuminous or nitrogenous 

 nature), correspond in position to the bast portion of the 



