496 POPULAR SCIENCE MONTHLY. 



matter, and the way is blocked ; the canal is filled with debris, so to say, 

 and has become disused. Again, the old central wood frequently decays 

 until there is only the outer ring of the later-formed wood remaining 

 with the bark that covers it. That the bark is not the water-carrier 

 may be shown by removing a ring of it and thus breaking the con- 

 nection without interrupting the upward flow. That it does pass 

 through the young wood may be shown by cutting this portion without 

 harming materially the bark or the heart wood, when the leaves quickly 

 wither and the tree may die. In short, the sap-wood is well named, as 

 through it the soil-water mounts upward from the roots to the leaves. 



In many plants, however, there is no well-developed ring of wood. 

 Either the stem is too young to have one or its construction such 

 that it never appears. However, the same kind of tissue is somewhere 

 to be found in the stem, usually in strands or portions of tough 

 threads, as in the corn-stalk, and through these the crude sap is trans- 

 ported. Some of these succulent stems are so transparent that they 

 admit of experiments which demonstrate both the path and the rate of 

 the upward flow. For example, a balsam stem may be cut and, while 

 fresh, plunged into a harmless colored liquid, as that of some aniline 

 dye. It is found that the woody bundles are the flrst to take the stain 

 and that it mounts upward with a rate that is an index of the flow of 

 sap and may be some feet in a single hour. Another test for the rate is 

 found in the use of a harmless salt, easily detected in extremely minute 

 quantities by the spectroscope. Let it be lithium nitrate, for example, 

 and its rise discovered by making sections of the stem at different dis- 

 tances and burning small fragments. 



But having determined the place of entrance, line of ascent and 

 point of departure of the aqueous stream, it by no means follows that 

 all the forces have been named that bring about the transfer. That 

 living plants carry water and make it one of the chief labors of all 

 their active days is beyond question, but physicists and physiologists, 

 chemists and biologists are as one concerning the mystery tliat here 

 exists. A grape-vine stump bleeding in early spring is a stumbling 

 block for them all, and they fall back upon 'root pressure,' a term more 

 convenient for covering much ignorance than for service as a full, 

 well-rounded explanation of the phenomena in question. Membrane 

 diffusion will account for much, capillary attraction helps considerabl}'-, 

 and the differences of gas pressure within and without the cells, as in 

 ihe tapped sugar maple in early spring, count for something ; but back 

 of all is a vital force that has not been reduced to a physical or chemical 

 basis. 



