IIMTION.J SP1KAL VESSELS. UoTHREXCHYM. 177 



from the cambium, were filled with air. Indeed, I must 

 confess, that I cannot conceive how any one, who has ex- 

 amined a great number of plants with attention, and only 

 applies sound logic, can set up the doctrine, that the spiral 

 vessels, and the woody fibres associated with them, are in- 

 tended to carry fluid. Never and nowhere is a fluid found 

 in them, excepting during a short time in the spring, in the 

 forest trees of our own climate, which may be accounted for 

 very simply, by the superabundance of the rising sap, and 

 the permeability of the cell-membrane ; but this being only 

 a periodical phenomenon, belongs as little to the usual course 

 of vegetation, as the human uterus can be said to be a blood- 

 vessel on account of its menstruating. A considerable 

 quantity of fluid flows out rapidly from the cut stem of the 

 Hoya carnosa in our hothouses, but the microscope instantly 

 shows, that all the spiral and porous vessels carry only air. 

 The answer derived from the rapidity of the flowing out of 

 sap is not worth much ; for every botanist knows, or may 

 readily convince himself, by placing a slice of potato under 

 the microscope, and adding a drop of tincture of iodine, when 

 it progresses as rapidly through the walls of the cell as on 

 the table ; therefore the living membrane of the cell offers 

 little or no resistance to the absorption of fluids. In the 

 same way as inorganic substances are permeable (most of the 

 perfect crystals, at least of the alkalies and earths) to the 

 imponderables, light, warmth, &c., so also is the organic 

 substance permeable for fluids. It is not the passing through 

 of a fluid, which is the effect of a vital power, which requires 

 explanation, but quite the reverse ; it is the retention of the 

 fluids in certain cells, which retention either originates from 

 a particular organisation, as in the epidermis, or from the 

 difference of the medium on both surfaces (air and fluid), as, 

 for instance, in the air cells, or from peculiar organic powers, 

 as, for instance, in the cells with coloured juices, existing 

 between the cells with uncoloured juices. Since the lifeless 

 vegetable membrane retains fluids, the most simple method 

 is to attribute this as a primary quality also to the living 

 membranes, and to search only for particular powers when 

 they allow a fluid to pass through them. The juice which 



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