OK. If I can remember correctly what Dr. Hamilton taught us in Wood Tech class 43 years ago (God but I'm getting old!), here are a few basics that we can think about when selecting stock blanks: (I'm talking about hardwoods and not conifers, and north of about latitude 20 degrees).
1. Wood strength is correlated with the percent of springwood to summerwood. When the tree breaks dormancy in the spring and initiates growth, the springwood cells are laid down first. These cells are larger in diameter than the later summerwood cells, and are weaker in strength. Therefore, the higher the percentage of springwood cells to summerwood cells, the weaker the wood and vice versa. A 10 power hand lens will show you the difference in these cells.
2. The higher the percentage of springwood to summerwood, the more stable the sawn lumber will be. While density will be greater with thick walled summerwood cells, so too will the potential for warpage of sawn lumber ie stock blanks. The denser the wood, though, the stronger it is per cubic increment of measure. So in effect it can be a trade-off....strength or possible warping. However! If hardwood is properly dried and seasoned, warping should be almost non-existing. The danger lies in poorly kiln dried hardwood blanks or lumber, and believe me....kiln drying hardwoods is more than an art and a science! The man who operates the kiln with hardwood is the most important man in the lumber manufacturing operation!
3. The color differentation between sapwood and heartwood is caused by food storage within the tree. Food material is manufactured within the leaves and transfered down to the root system where it is stored for next years' growth. As a tree ages the crown-the leaves-manufacture more food material than the roots can store, so the excess is transfered to the cells making up the woody part of the trunk. This transfer is made through a system of cells called "storied ray cells" which run from the living portion of the tree just under the bark to the very center or heart of the tree. As the tree ages more food material is deposited and the color changes to a darker color. The lighter sap wood is simply the newer wood or cells, and hasn't filled up with food material yet.
4. IIRC the "curl" that we all want in our stocks is caused by the food material within the storied ray cells being transfered to the interior cells. These cells are at right angles to the other up and down oriented cells, and are full of darker food material and stand out when the log is sawn. Usually age determines the amount of curl, but not always so even with younger trees it can be seen. But usually curl is much more common in older ones.
Newer information may have developed since I was exposed to wood tech and there may be different opinions on this now. But this info should be about right I think.