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(letcott'd l)\- careful (.•xamination. In most trees the inner part 

 of the \V(U)(1 and the pith are dead and this is called the heart 

 wood or duramen, while the outer wood is li.Lihter in color 

 with living cells next the caniliium 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 arc called medullary rays. The i)eculiar (jualities of 

 wood are due t(^ the character of its cells which have their 

 walls lignitied or thicki-ned by a dei)'>sit of a chemical 

 substance called lignin. 



The bark usually consists of two main la\ers called inner 

 bark and outer bark. IMie inner liark is often in very thin 

 layers and is lience called liber. The outer bark is very 

 diverse in character. Usually it consists mainh' of la\ crs of 

 cork' cells which are \ery impervious to water. Since the 

 outer bark usually docs not increase in diameter as rapidly 

 as the wood it is linally torn into strips and peels off on the 

 outside. Trees have many interesting wa\s of de\eloping and 

 getting rid of their outer bark. 



More commonly the outer bark is developed as follows: 

 In a 3'ouiig main stem or twig there is a tissue between the 

 outer layer of cells or epidermis andthc circle of vascular 

 bundles which is called the cortex. While the stem is de- 

 veloping and hardening, the outermost layer of cortical cells 

 just below the epi.dermis is modified and begins to grow. 

 This layer is the cork camiliium or phellogen. 1 he layer of 

 tissue thus formed by the repeated divisions of the cells of 

 the phellogen is called the 'periderm or cork, (hi the inner 

 side of the phellogen another layer of tissue is produced 

 whicii is called the phelloderm or secondary cortex. The 

 l)liellogen may continue to produce i)eriderm until the outer 

 bark becomes \ery thick : and linally new cork cambiums nia\ 

 develop farther in in the cortex or even in the phloem of 

 tile inner bark. In some plants the cork cambium originates 

 from the epidermis and in some from the deeper layers of 

 I lie cortex. 



Some trees have no special means oi shedding their leaves 

 while others shed them only after a year i^r more. Most of 

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

 leaxes at the end of each growing season b>" the formation 



