The disadvantages of fall sowing are also many.  The uncertainty of the fall rains must first be considered.  In ordinary practice, seed sown in the fall does not germinate until a rain comes, unless indeed sowing is done immediately after a rain.  The fall rains are uncertain as to quantity.  In many cases they are so light that they suffice only to start germination and not to complete it and give the plants the proper start.  Such incomplete germination frequently causes the total loss of the crop.  Even if the stand of the fall crop is satisfactory, there is always the danger of winter-killing to be reckoned with.  The real cause of winter-killing is not yet clearly understood, though it seems that repeated thawing and freezing, drying winter winds, accompanied by dry cold or protracted periods of intense cold, destroy the vitality of the seed and young root system.  Continuous but moderate cold is not ordinarily very injurious.  The liability to winter-killing is, therefore, very much greater wherever the winters are open than in places where the snow covers the ground the larger part of the winter.  It is also to be kept in mind that some varieties are very resistant to winter-killing, while others require well-covered winters.  Fall sowing is preferable wherever the bulk of the precipitation comes in winter and spring and where the winters are covered for some time with snow and the summers are dry.  Under such conditions it is very important that the crop make use of the moisture stored in the soil in the early spring.  Wherever the precipitation comes largely in late spring and summer, the arguments in favor of fall sowing are not so strong, and in such localities spring sowing is often more desirable than fall sowing.  In the Great Plains district, therefore, spring sowing is usually recommended, though fall-sown crops nearly always, even there, yield the larger crops.  In the intermountain states, with wet winters and dry summers, fall sowing has almost wholly replaced spring sowing.  In fact, Farrell reports that upon the Nephi (Utah) substation the average of six years shows about twenty bushels of wheat from fall-sown seed as against about thirteen bushels from spring-sown seed.  Under the California climate, with wet winters and a winter temperature high enough for plant growth, fall sowing is also a general practice.  Wherever the conditions are favorable, fall sowing should be practiced, for it is in harmony with the best principles of water conservation.  Even in districts where the precipitation comes chiefly in the summer, it may be found that fall sowing, after all, is preferable.