Proceedings of the Ohio State Academy of Science 77 



can usually be detected by careful examination. In most trees 

 the inner part of the wood and the pith are dead and this is called 

 the heart wood or duramen, while the outer wood is lighter in 

 color with living cells next the cambium layer, and is called the 

 sap-wood or alburnum. Sometimes there is a striking difference 

 in the color of the two parts. Strands of cells pass from the 

 pith or annual rings through the wood to the bark. These are 

 called medullary rays. The peculiar qualities of wood are due 

 to the character of its cells which have their walls lignified or 

 thickened by a deposit of a chemical substance called lignin. 



The bark usually consists of two main layers called inner 

 bark and outer bark. The inner bark is often in very thin lay- 

 ers and is hence called liber. The outer bark is very diverse in 

 character. Usually it consists mainly of layers of cork cells 

 which are very impervious to water. Since the outer bark usually 

 does not increase in diameter as rapidly as the wood it is finally 

 torn into strips and peels off on the outside. Trees have many 

 interesting ways of developing and getting rid of their outer bark. 



More commonly the outer bark is developed as follows : In 

 a young main stem or twig there is a tissue between the outer 

 layer of cells or epidermis and the circle of vascular bundles 

 which is called the cortex. \Miile the stem is developing and 

 hardening, the outermost layer of cortical cells just below the 

 epidermis is modified and begins to grow. This layer is the cork 

 cambium or phellogen. The layer of tissue thus formed by the 

 repeated divisions of the cells of the phellogen is called the peri- 

 derm or cork. On the inner side of the phellogen another layer 

 of tissue is produced which is called the phelloderm or secondary 

 cortex. The phellogen may continue to produce periderm until 

 the outer bark becomes very thick ; and finally new cork cam- 

 biums may develop farther in in the cortex or even in the phloem 

 of the inner bark. In some plants the cork cambium originates 

 from the epidermis and in some from the deeper layers of the 

 cortex. 



Some trees have no special means of shedding their leaves 

 while others shed them only after a year or more. Most of our 

 indigenous species are "deciduous," that is they cast their leaves 



