102 THE STRUCTURE AND WORK OF THE STEM 



roots to the leaves and from the leaves to the stem and root. The position of 

 the leaf traces on the branch give us a clew as to the appearance of the 

 leafy tree. If the leaf traces are oppositely placed, then we know that 

 the leaves and buds, which give rise to lateral branches, had a very 

 definite arrangement in pairs. Such are the maple or horse-chestnut. 

 If, on the other hand, the leaf traces are placed alternate to each other, 

 we can picture a tree with much less regularity in the position of leaves 

 and lateral branches, as in the apple, beech, and elm. 



Four years' growth in an ailanthus stem, showing the changes in the lenticels from 

 round holes to elongated cracks in the bark. The lenticel in a young shoot is 

 like the breathing hole of a leaf. 



Lenticels and their Uses. The very tiny scars, which look like little 

 cracks in the bark, are very important organs, especially during the winter 

 season, for they are the breathing holes of the tree. A tree is alive in 

 winter, although it is much more active in the warm weather. Oxidation 

 takes place much more rapidly in the summer because the plant is grow- 

 ing rapidly, and more fuel is consumed to release the energy needed for 

 growth. We shall see later that the leaves are the chief breathing organs 

 of the plant. But all the year round oxygen is taken in and carbon 

 dioxide given off through the lenticels, as the breathing holes in the trunk 

 and branches of a tree are called. The lenticels, which early in the life 

 of the stem are structures similar to the breathing holes in leaves (of which 

 more later) , become quite changed in older stems, the tiny holes becoming 

 cracklike scars. 



A Dicotyledonous Stem in Cross Section. If we cut a cross sec- 

 tion through a young horse-chestnut stem, we find it shows three 



