All nut growers are confronted with the problems involved in the production of nuts of large size with well filled kernels that are "bright" or light colored. Unsatisfactory development and filling of the kernels is more often a cause of complaint by growers than any other single factor affecting nut production. This is because all of our commercial nuts now sold in the shell are priced on a basis of size and the degree to which they are filled. The size and degree of filling of the nuts varies not only from year to year, but from district to district, orchard to orchard, and even in the same orchard, because nuts of one variety may fill well and those of another poorly. This is true even though the kind and variety of nut being produced is grown in a locality usually having suitable climatic conditions for normal nut production. +Climatic Conditions+ Prevailing climatic conditions in any locality determine how well a particular kind of nut will fill. For example, the pecan is native to the southern part of the United States and a small area in northern Mexico. In its native habitat the summers are long and the day and night temperatures are uniformly high, with little difference between maximum and minimum daily temperatures. When the pecan is grown under conditions of shorter summers, or where there is a marked difference between night and day temperatures, the nuts do not grow to proper size and the kernels fill poorly, if at all. Although pecan trees are quite hardy and may be grown successfully well north of their native limits, the normal development of the nuts and the filling of them cannot be expected there. Good examples of the climactic effects can be cited. At Davis, California, the pecan tree grows, flowers, and sets fruit satisfactorily, but the nuts fail to grow to proper size, fill poorly, and may not mature before frost. At Davis there is an average length of growing season of 242 days; the day temperatures are high, but the night temperatures are comparatively low. Pecan trees are hardy even in Connecticut, but the trees fail to bear because of the short growing season and the great difference between day and night temperatures. The pecan is truly a hot weather crop and is not suited for culture under mountainous conditions. On the other hand it cannot be grown under subtropical conditions because of insufficient cold during the winter to meet the chilling requirement of the trees. Under such conditions, tree growth starts very late in the spring, and, although the trees may flower, few nuts may set and those that stick may be very poorly filled at harvest if they mature.