THE WORK OF STEMS 103 



we are able to tell the age of the tree, limb or twig by- 

 counting the rings in its cross section. 



79. How Food travels from Leaves to Roots. — The 

 food made in the leaves has three uses. (Section 70.) 

 In order that it may be used in any of these ways, it must 

 first be conducted from the leaves to the different parts 

 of the plant. We have already learned that it takes 

 several hundred pounds of water to produce one pound of 

 dry matter or food material in the plant. (Section 69.) 

 It is evident, then, that the vessels through which the 

 manufactured food travels from the leaves to other parts 

 of the plant need not be so large as those by which the soil 

 water travels from the earth to the leaves. The tubes, 

 called sieve tubes, which carry the manufactured food to 

 different parts of the plant, are very small indeed ; and 

 no careful study can be made of them without specially 

 prepared material and a compound microscope. It is 

 enough for us here to know that in the dicotyledons, these 

 sieve tubes are outside of the water-carrying vessels just 

 beneath the bark; while in the monocotyledons, they 

 are connected with the water-carrying vessels, forming the 

 fibro-vascular bundles, as explained in Section 77. These 

 bundles, which are seen as threads, running through the 

 pith of the cornstalk, have the double function, then, of 

 carrjdng water from the soil to the leaves, and manufac- 

 tured food from the leaves back to other parts of the plant. 



You have perhaps noticed that where a notch has been 

 cut in a tree, a callus is formed above, but not below, the 

 cut. This means that the food on its downward path has 

 found a place where the " bridge is out " and has piled up 

 on the bank on the side from which it has come. Girdling 

 a tree is simply the cutting away of the bark and the sieve 

 tubes just beneath it. This stops the current of food and 



