Between A and B arrows is one growth increment in girth (oak stem).  We breakdown the increments to two stages.  Springwood, or also known as, Earlywood and Summerwood also known as Latewood.  Letter C points to the wood formed in the Spring and Letter D points to the wood formed in the summer.  This sample is sapwood and we call it non-conducting sapwood as you can see the vessels are plugged with tyloses and will not conduct liquids.  Trees plug vessels at least 7 different ways.  Notice the stored starch.  Lots of money in the bank.  This sample was stained with iodine in potassium iodide solution which turned the starch grains purple to black.  Every place you see black you have living cells.  The plugging of the vessels with tyloses is common in white oak which will have only one increment of conducting sapwood.  Red oak forms what we call traumatic tyloses when wounded.   (Picture - SHIGO, 2002)

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Source SHIGO 2002 CD's

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