The forms of organisms are evidently often directly related to surrounding influences. A plant or plant-like animal fixed to the soil may be so fixed that light, air, food, friends and enemies can have access equally on all sides or not. Thus, a tree so placed that light and air are excluded on one side, will not grow freely towards that side, but only in directions from whence light and air have access. A coral reef increases much more rapidly towards the open sea (the waves of which bring in food and facilitate gaseous interchange) than towards an adjacent shore.
The mere contiguity of parts will often affect the form of organisms. Thus, in many flowers parts which are adjacent become dwarfed, while others which are freely exposed become fully developed, as we see in the flowers of many _Umbelliferæ_, or plants of the parsley, fennel, and hemlock order.
The shapes of flowers bear relation (as we shall see later) to their need for attracting insects which by their visits effect the development of seed, and for repelling others the access of which would be hurtful.
The avoidance of enemies may be so effected by an organism that their access may be made impossible save in one direction, the extent of vulnerable surface even in that direction being minimized. We have an example of such a condition in those worms which live in calcareous tubes, and which are some of those called "tubicolous annelids."[60]
Again, the medium in which an organism lives--whether aërial or aqueous--has an important relation with its form. A delicate seaweed, the beautifully radiating form of which is a just object of admiration as long as it is supported by its denser natural medium (the sea water), collapses into an amorphous mass when withdrawn thence into the thin air. Obviously a much greater rigidity and strength of structure is needed to support an aërial organism than an aquatic one, unless the former can support itself on other solid structures, such as rocks or trees. In the latter case the form attained may be very elongated and slender, as in the many creeping and climbing plants, which are so often furnished with processes for grasping (tendrils) to aid them in their mode of life.