THE LIFE HISTORY OF A TREE 13 



live without it, as is shown by the fact that hollow trees 

 can live to a great age. 



These pipes and cells are of many shapes, and are arranged 

 in many different ways in the several species. 



On cutting across the stem of a tree we see the annual 

 rings. These are formed in the following way : In the spring 

 when the leaves sprout there is a great demand for water, 

 and the pipes and cells which are then formed have thin walls 

 and large openings. Later on in summer there is not so 

 much want of water, and smaller openings will do ; at the 

 same time the bark begins to press on the wood, so that the 

 cell walls must be capable of resisting this pressure ; therefore, 

 in summer the pipes have thick walls and small openings. In 

 winter growth stops, and it begins again next spring with the 

 pipes with large openings. These being formed next to the 

 thick-walled ones give the appearance of a ring on a cross 

 section. 



These rings are well defined and easily seen in the timber 

 of conifers. In that of broad-leaved trees, though the general 

 manner of formation of the rings is the same, there are slight 

 differences in detail which result in the rings being rather less 

 sharply marked. 



Usually one ring is formed each year, so that the age of the 

 tree is known by counting the rings on a section cut at the 

 bottom of a stem. Under certain circumstances, however, 

 such as when the leaves are destroyed by caterpillars in early 

 summer, followed by a second flush, two rings may be formed 

 in a year. 



Flowering plants are divided into three great classes : 

 dicotyledons, which have two seed leaves ; monocotyledons, 

 which have one seed leaf; and gymnosperms, which 

 differ in many important respects from the other two, 

 though for timber purposes they may be compared with 

 dicotyledons inasmuch as their wood is also characterized by 

 annual rings. The structure of the stem of a monocotyledon 



