=899. The virgin’s bower.=—The virgin’s bower (Clematis virginiana), too, clambering over fence and shrub, makes a show of having transformed its exquisite white flower clusters into grayish-white tufts, which scatter in the autumn gusts into hundreds of arrow-headed, spiral plumes. The achenes have plumose styles, and the spiral form of the plume gives a curious twist to the falling seed (fig. 485). [Illustration: Fig. 485. Seed distribution of virgin’s bower (clematis).] CHAPTER XLVI. VEGETATION IN RELATION TO ENVIRONMENT.[47] I. Factors Influencing Vegetation Types. =900.= All plants are subject to the influence of environment from the time the seed begins to germinate until the seed is formed again, or until the plant ceases to live. A suitable amount of warmth and moisture is necessary that the seed may germinate. Moisture may be present, but if it is too cold, germination will not take place. So in all the processes of life there are several conditions of the environment, or the “outside” of plants, which must be favorable for successful growth and reproduction. Not only is this true, but the surroundings of plants to a large extent determine the kind of plants which can grow in particular localities. It is also evident that the reaction of environment on plants has in a large measure caused them to take on certain forms and structures which fit them better to exist under local conditions. In other cases where plants have varied by mutation (p. 338) some of the new forms may be more suited to the conditions of environment than others and they are more apt to survive. These conditions of environment acting on the plant are _factors_ which have an important determining influence on the existence, habitat, habit, and form of the plant. These factors are sometimes spoken of as _ecological factors_, and the study of plants in this relation is sometimes spoken of as ecology,[48] which means a study of plants in their home or a study of the household relations of plants. These factors are of three sorts: 1st, physical factors; 2d, climatic factors; 3d, biotic factors.