Unlike softwoods, most hardwoods lack resin canals (excepting some tropical trees with gum ducts). They have much thicker rays and are generally a more complex wood. It is speculated that because hardwoods are so much more complex and have specialized parts that perform conduction and support functions, they most likely evolved after softwoods. These specialized cells are called vessel elements, and connect end to end within the wood forming a sort of straw by which sap is conducted through. When these vessels are cut across the end grain, the wood's pores are revealed.
Because only hardwoods have these vessels, they are sometimes referred to as porous woods (and softwoods as nonporous). These vessels help to determine the appearance, strength, and hardness of the wood, depending on their arrangement, size, and amount. Some woods have fibers which are smaller in diameter and have closed ends making for poor sap conduction. If more pores are concentrated within the earlywood, it is considered to be a ring-porous wood, which has a noticeably uneven grain although some ring-porous woods are not terribly affected in terms of their density. This type of wood would most likely not be the best choice for hardwood flooring. When it comes to even pore distribution, the wood is considered to be diffuse-porous. Woods like maple, birch, and basswood fall in this category and would make a wonderful choice for wood floors because they are very hard and resistant. Keep in mind that they are not a good candidate for a stain color, however.
Some woods fall in between these two, however. Semi-ring-porous woods, like black walnut and butternut, have large pores in their earlywood, which get smaller as they get closer to the latewood. What makes it important for you to know what kind of pores your wood has? Well, the pores determine the texture of your wood floor.
Some hardwoods even have tyloses, a bubblelike structure at the cavities of vessel elements in the sapwood to heartwood transition. Depending on what wood you're working with, these can be non-existent, sparse, unevenly distributed or densely packed. No generalization can be given to a hardwood's rays – these vary from a single cell wide to 40 cells wide and thousands of cells high. When a ray adds a distinct look to the edge grain of the wood, it is often referred to as ray fleck and is used for aesthetic purposes. Ray flecks can add a special flavor to any wood floor, making it uniquely your own.