Yearbook, of Agriculture 1949 



material to be used by the germinating 

 embryo. Proteins are formed again 

 from the amino acids, and dextrose is 

 partly used for building the body of the 

 tree and partly burned up to provide 

 necessary energy for the process. 



The embryo grows fast. Soon the 

 seed shell becomes too small and splits 

 open. The newly born tree emerges 

 above the ground. Its shoot begins to 

 grow straight up and its roots straight 

 down. The root has important work to 

 do; it provides water for the young 

 seedling. As soon as the little root of a 

 seedling penetrates the ground, the tree 

 is permanently anchored, for better or 

 for worse, to the place where, unless 

 it is transplanted, it has to stay all its 

 life. From now on the tree has to de- 

 pend on the nutrients available in that 

 particular place and to develop under 

 climatic conditions found there, which 

 cannot be changed. In nature, how- 

 ever, a seedling generally begins its life 

 in a place where its ancestors have been 

 growing for a long time, so the little 

 tree is well adapted to the existing 

 conditions. 



As it emerges from the ground, a 

 young tree seedling is as tender as a 

 blade of grass. Its seed leaves may 

 remain in the shell below the ground, 

 as in oak, or they may be carried above 

 the ground, as in maple. In pine, the 

 seed leaves pull themselves out from 

 the endosperm and spread above the 

 seedling like the crown of a miniature 

 palm tree. On the tip of the little stem, 

 tucked between the seed leaves, is the 

 growing point or terminal bud that 

 gives origin to the shoot; its growth 

 continues as long as the tree lives. 



Besides the root and stem tips, an- 

 other important growing region is soon 

 established in the seedling. It is called 

 the cambium layer and is found be- 

 tween the wood and the bark. It makes 

 the tree grow in girth. The cambium 

 consists of a single layer of cells that 

 retain their capacity to divide through- 

 out the life of the tree. This single 

 layer of cells has a peculiar property 

 in that it gives origin both to the wood 

 and to the bark. In the spring, when 



the cambium layer becomes active, it 

 begins to split off rows of wood cells 

 to the inside and rows of bark cells 

 to the outside. Generally speaking, the 

 bark part of the tree is much thinner 

 than the woody part, or the stem. Bark 

 continuously sloughs off, while the 

 wood accumulates. In the soft inner 

 bark, or bast, are formed sieve tubes, 

 through which manufactured sugar 

 dissolved in water flows from the foli- 

 age to storage tissues in stem and root. 



The wood formed in the spring con- 

 sists of light-colored, thin-walled cells ; 

 toward the end of the season smaller 

 cells are formed their walls are heav- 

 ier and darker, and thus summer wood 

 is formed. This alternation of spring 

 wood and summer wood causes the 

 concentric structure of the tree trunk 

 known as annual rings; they are seen 

 clearly on the cross section of a tree. 

 By counting the annual rings of a tree, 

 one can determine fairly closely its age. 

 When growth conditions are favorable 

 and food and water are abundant, the 

 rings are wide. When drought occurs, 

 the growth slows down and the rings 

 are narrow. By reading a cross section 

 of an old tree, one can determine 

 what growth conditions prevailed dur- 

 ing any particular year of the past. 



In the cross section of the hardwood 

 trees there may be seen numerous dots. 

 These are canals, so-called vessels, that 

 serve for conducting water along the 

 trunk. In the conifers, like pines or firs, 

 there are no vessels and water moves 

 painstakingly up the trunk through 

 minute holes from one cell to another. 



Sixty percent of the wood of a tree 

 is cellulose by far the most important 

 ingredient. The structure of cellulose 

 is well understood and is rather simple : 

 Molecules of dextrose are linked in 

 pairs to form a more complex sugar, 

 cellobiose, and these units are hooked 

 up to form long chains of cellulose 

 molecules. This structure of cellulose 

 may be easily changed by action of 

 even a weak acid; cellulose then falls 

 apart into the original dextrose mole- 

 cules, providing an enormous source of 

 sugar that can be used for many pur- 



