=715. Columnar type.=—The columnar type of stem may be simple or branched. When branching occurs the branches are usually small and in general subordinate to the main axis. The sunflower (Helianthus annuus) is an example. The foliage part is mainly simple. The main axis remains unbranched during the larger part of the growth-period. The principal flowerhead terminates the stem. Short branches bearing small heads then arise in the axils of a few of the upper leaves. In dry, poor soil, or where other conditions are unfavorable, there may be only the single terminal flowerhead, when the stem is unbranched. The mullein is another columnar stem. The foliage part is rarely branched, though branches sometimes occur where the main axis has become injured or broken. The flower stem is terminal. The corn plant and the Easter lily are good illustrations also of the columnar stem. Among trees the Lombardy poplar (Populus fastigiata) is an excellent example of the columnar type. Though this is profusely branched, the branches are quite slender and small in contrast with the main axis, unless by some injury or other cause two large axes may be developed. As the technical name indicates, the branching is fastigiate, i.e., the branches are crowded close together and closely surround the central axis. The royal palm and some of the tree ferns have columnar, simple stems, but the large, wide-spreading leaves at the top of the stem give the plant anything but a cylindrical habit. Some cedars and arbor-vitæ are also columnar. The advantages of the columnar habit of stem are three: (1) That the plant stands above other neighboring ones of equal foliage area and thus is enabled to obtain a more favorable light relation; (2) where large numbers of plants of the same species are growing close together, they can maintain practically the same habit as where growing alone; (3) the advantage gained by other types in their neighborhood in less shading than if the type were spreading. The cylindrical type can, therefore, grow between other types with less competition for existence. [Illustration: Fig. 422. Cylindrical stem of mullein.] [Illustration: Fig. 423. Conical type of larch.]