How do timber cells affect the appearance of timber grain?

IF the cells of the wood are uniform, they have a uniform appearance; any visible defect (many of which appear after drying) will cause uneven gain, drying marks, fungal or other damage to the appearance of the wood.[1] [2]

Most common of these are sticker stain, which appears in wood that has been dried incorrectly.[3] [4]

(Image from Popular woodworking)

When finishing wood that has defects, and won’t take stain readily, you sometimes have to use a transparent tinting stain – one that evenly covers the wood. Many species don’t finish well, and this give you a uniform finish. You would need a tint like this:
or this

This is one of my favorites:

When finishing woods that have a high sap content, like Pine (or most of the coniferous woods) you may need to coat them with a pre-finish, so they will take stain more easily.  
Minwax 61500444 Pre Stain Wood Conditioner, 1 Quart
Always test a scrap piece of wood to ensure you get the finish right!  And remember to keep rags and cleaner like Mineral spirits or Lacquer Thinner handy.  Make sure your finishing area is well ventilated, and you clean the surface after sanding with a tack cloth.
Many grain defects don’t show up to the naked eye until you add a stain or finish coat.  
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This is expanded from my answer that originally appeared in Quora on June 29, 2017
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