The wheat plant has long slender leaves and stems that are hollow in most varieties. The inflorescences are composed of varying numbers of minute flowers, ranging from 20 to 100. The flowers are borne in groups of two to six in structures known as spikelets, which later serve to house the subsequent two or three grains produced by the flowers. Though grown under a wide range of climates and soils, wheat is best adapted to temperate regions with rainfall between 30 and 90 cm (12 and 36 inches). Winter and spring wheat are the two major types of the crop, with the severity of the winter determining whether a winter or spring type is cultivated. Winter wheat is always sown in the fall; spring wheat is generally sown in the spring but can be sown in the fall where winters are mild The nutritional composition of the wheat grain varies somewhat with differences in climate and soil. On an average, the kernel contains 12 percent water, 70 percent carbohydrates, 12 percent protein, 2 percent fat, 1.8 percent minerals, and 2.2 percent crude fibres. Thiamin, riboflavin, niacin, and small amounts of vitamin A are present, but the milling processes removes most of those nutrients with the bran and germ Most wheat used for food requires processing. The grain is cleaned and then conditioned by the addition of water so that the kernel breaks up properly. In milling, the grain is cracked and then passed through a series of rollers. As the smaller particles are sifted out, the coarser particles pass to other rollers for further reduction. About 72 percent of the milled grain is recovered as white flour. Flour made from the whole kernel is called graham flour and becomes rancid with prolonged storage because of the germ-oil content retained. White flour, which does not contain the germ, preserves longer. Inferior and surplus wheats and various milling by-products are used for livestock feeds. The greatest portion of the wheat flour produced is used for breadmaking. Wheats grown in dry climates are generally hard types, having protein content of 11–15 percent and strong gluten (elastic protein). The hard type produces flour best suited for breadmaking. The wheats of humid areas are softer, with protein content of about 8–10 percent and weak gluten. The softer type of wheat produces flour suitable for cakes, crackers, cookies, and pastries and household flours. Durum wheat semolina (from the endosperm) is used for making pastas, or alimentary pastes.