104 GENERAL BOTANY 



necessary for the growth of a young tree from the seed is thus 

 compensated for by its extraordinary life period when it is once 

 established. 



Physiology of the tree. During the summer months water is being 

 absorbed by the roots and passed up the great water ducts to the 

 living tissues of the tree trunk and to the leaves, where a small 

 part is used in the manufacture of food and a very large part is 

 evaporated. During the entire summer season the leaf is making 

 food in the form of sugar, starch, and soluble nitrogenous food sub- 

 stances. A part of this food is used at once by the living tissues of 

 the leaf, but most of it is transported back through the phloem into 

 the stem and stored in the wood rays and the wood parenchyma. 

 The food moves down the veins and vascular tissue of the leaf 

 petiole into the phloem of the branches and main tree trunk. In the 

 phloem the sieve tubes serve to convey the greater part of the nitrog- 

 enous food materials, while the sugars move in the phloem paren- 

 chyma and companion cells of the sieve tubes. When the food 

 stream reaches the wood rays in the phloem, a portion of it passes 

 horizontally along the rays to the xylern, where it is usually trans- 

 formed into starch and stored in the rays themselves and in the 

 living wood-parenchyma cells of the sapwood. If the tree is young 

 and still retains a living pith and cortex, a portion of the food may 

 pass into these tissues and be stored. Trees in which the stored 

 food is mainly in the form of starch are often called starchy trees, 

 to distinguish them from trees like the linden, or basswood, in which 

 the reserved food is mainly composed of fats. In either case the 

 reserve food is converted into sugar, by digestion, before it is circu- 

 lated and used for growth and repair. When spring growth begins, 

 before the advent of the leaves, the food currents are reversed and 

 pass outward from the rays and wood parenchyma to supply the 

 cambium for its growth. In a similar manner food streams move 

 upward and downward in the phloem to supply the growing buds 

 and root tips with food at the extremities of the tree. We see, there- 

 fore, that the tree, while its tissues provide adequately for the trans- 

 portation of water and foods and for food storage, has no circulation 

 such as that provided by the heart, arteries, and veins of the higher 

 animals. Foods move up or down, outward or inward, according to 

 the needs of the tissues in any given part. Water, on the contrary, 

 moves mainly upward, although some lateral movement is necessary 

 in order to supply the living cells of wood, phloem, pith, and cortex. 



