According to Alfred Rehder, of Harvard, in the Standard Cyclopedia of Horticulture, six species of hickory are indigenous to that region east of the Rocky Mountains here discussed under the term of the northernmost nut zone. These are the shagbark, the shellbark, the sweet hickory, the pignut, the mockernut and the bitternut. The shagbark hickory, Hicoria ovata, and the sweet hickory, H. ovalis, are the principal ones among this group offering promise as sources of varieties fit for cultivation in this zone. The former is well known as a rich-land species, having shaggy bark and a more or less sharply angled sweet nut; the latter, often called pignut, has recently been listed as "sweet hickory" to distinguish it from H. glabra, also called pignut, yet which is sometimes better. The sweet hickory is less exacting in soil requirements than the shagbark, although often nearly or quite as good a nut, popular prejudice notwithstanding. When shelled the kernels can be distinguished only with difficulty. Of the other hickories indigenous to this zone, all are omitted from the discussion for definite reasons, chief of which is the fact that few or no seedlings of promise have been found. The shellbark, H. laciniosa, which is much like the shagbark in many respects, occurs in this zone sparingly and only in the southernmost part. Nuts of this species, while very large, are thick-shelled and commonly more or less objectionable because of the frequency with which the kernels are imperfectly developed or entirely wanting. The pignut hickory, H. glabra, already mentioned, is omitted from further discussion because of being no better than the sweet hickory in any known respect, and because of the frequent bitterness of its kernel. The mockernut, H. alba, while indigenous practically everywhere that any other hickory grows, and producing a sweet, agreeable kernel, has too thick a shell to justify particular attention at this time. The bitternut hickory, H. cordiformis, is rarely palatable. The tree makes an attractive ornamental, but is relatively unimportant in so far as timber production is concerned. Intermediate forms of hickory and hybrids originated from chance crosses under purely natural conditions are fairly common. Quite a good many belonging to one or the other of these groups have been brought to light during the last two decades, largely as a result of discovery by the Northern Nut Growers Association. Several of these will be discussed in alphabetical order along with varieties of pure species.