As already stated, wood loses water faster from the ends than from the longitudinal faces. Hence the ends shrink at a different rate from the interior parts. The faster the drying at the surface, the greater is the difference in the moisture of the different parts, and hence the greater the strains and consequently also the greater amount of checking. This becomes very evident when freshly cut wood is placed in the sun, and still more when put into a hot, dry kiln. While most of these smaller checks are only temporary, closing up again, some large radial checks remain and even grow larger as drying progresses. Their cause is a different one and will presently be explained. The temporary checks not only appear at the ends, but are developed on the sides also, only to a much smaller degree. They become especially annoying on the surface of thick planks of hardwoods, and also on peeled logs when exposed to the sun. So far we have considered the wood as if made up only of parallel fibres all placed longitudinally in the log. This, however, is not the case. A large part of the wood is formed by the medullary or pith rays. In pine over 15,000 of these occur on a square inch of a tangential section, and even in oak the very large rays, which are readily visible to the eye, represent scarcely a hundredth part of the number which a microscope reveals, as the cells of these rays have their length at right angles to the direction of the wood fibres. If a large pith ray of white oak is whittled out and allowed to dry, it is found to shrink greatly in its width, while, as we have stated, the fibres to which the ray is firmly grown in the wood do not shrink in the same direction. Therefore, in the wood, as the cells of the pith ray dry they pull on the longitudinal fibres and try to shorten them, and, being opposed by the rigidity of the fibres, the pith ray is greatly strained. But this is not the only strain it has to bear. Since the fibres shrink as much again as the pith ray, in this its longitudinal direction, the fibres tend to shorten the ray, and the latter in opposing this prevents the former from shrinking as much as they otherwise would. Thus the structure is subjected to two severe strains at right angles to each other, and herein lies the greatest difficulty of wood seasoning, for whenever the wood dries rapidly these fibres have not the chance to "give" or accommodate themselves, and hence fibres and pith rays separate and checking results, which, whether visible or not, are detrimental in the use of the wood.