STEMS, KOOTS, AND LEAVES 99 



tissue connected with the wood rays and with the ducts. This 

 distribution of the starch-storage cells in the wood enables the 

 living cells to transport the sugar, which forms from the starch 

 in the spring, along almost continuous paths to the cambium 

 layer and the phloem. The other reserve foods, such as fats and 

 nitrogenous substances, are less easily demonstrated in wood, 

 but such reserves are laid up with the starch in trees in the thin- 

 walled tissues mentioned above. Toward spring, when the 

 growth of the tree begins, the solid reserves are converted into 

 soluble sugars, fats, and proteins, and migrate by osmosis to the 

 cambium, growing buds, and root tips, where they are converted 

 into protoplasm and cell walls. 



Conduction. The soluble foods just mentioned move horizon- 

 tally along the wood rays and upward or downward in the 

 wood parenchyma and the phloem, as the case may be. The 

 nitrogenous substances appear to move mainly in the sieve tubes, 

 while the soluble sugars migrate in the phloem parenchyma. 

 The method of movement is by osmosis, as already indicated, 

 and the direction may be upward toward the terminal buds, 

 downward toward the root tips, or outward into horizontally 

 placed branches and lateral buds. Mass movement also occurs 

 in sieve tubes when the bending of a tree by the wind squeezes 

 the sieve tubes and forces the food in them to move along. 

 Sugar also moves in the ducts in certain trees like the maple 

 and birch. In a tree or other plant there is not, therefore, any 

 definite circulation of foods, but rather a general movement 

 from places of storage to places of growth, or from places of 

 manufacture to storage tissues. The water and soil salts move 

 upward, in a stem, from the roots to the leaves. The main chan- 

 nels are the great water ducts, although in many trees some of the 

 fibrous cells, called tracheids, serve as conductors with the ducts. 

 In trees of the pine family these shorter wood cells (tracheids) 

 are almost the sole conductors of water. The great water 

 and food streams in a tree have therefore quite different paths, 

 so that they do not interfere with each other. In summer, when 

 the foods are being constructed in the leaves, the soluble foods 

 move downward in the phloem to be stored in the wood rays, 



