40 STEM AND BAKK OF PLANTS. 



be true of various kinds of food, as we find that far 

 more of one kind is taken than of another, even when 

 both are present in equal quantities. 



In the stem are numerous little tubes, running up 

 and down, which serve to convey the sap absorbed by 

 the roots up to the leaves. It passes up in the interior 

 vessels or tubes, and passes down in the exterior, or 

 just under the bark. This can be shown by the ex- 

 ample of the tree and the colored fluid, just referred 

 to; the inner part of the tree will be colored first, and 

 finally the outer, in the descent of the sap, after it has 

 passed out to the extremities of the branches. 



There is then a regular circulation between the soil 

 and the plant; sap flows up, having been formed in 

 the roots and stem, out of the various substances 

 drawn in from the soil, and ultimately flows down 

 again next the bark and out into the soil. 



During its circuit the sap undergoes many changes, 

 and deposits such of its constituents as are necessary 

 to the plant. If taken from the lower part of the 

 stem, it will be found thin; as it goes up, it appears 

 thicker and thicker, and at last on its w^ay down be- 

 comes a dense substance, to which the name of cam- 

 bium has sometimes been given. At this period of 

 its round, it deposites, between the inner bark and 

 the wood, material for forming the annual layer of 

 new wood. The cause of this ascent and descent of 

 sap is not fully known, and I do not consider it neces- 

 sary to mention here the numerous plausible theories 

 that have been advanced regarding it. If the flow is 

 entirely stopped, either upward or downward, the 

 plant soon dies. This is shown by the ordinary opera- 

 tion of girdling a tree, the downward flow is stopped 

 and no new wood can form. 



The hark is quite different in its structure from the 

 stem. In the latter part, as will be remembered, the 

 little tubes run perpendicularly, or straight up and 



