have only needle-shaped leaves or even no true ones at all, as many of the cacti in the desert lands of the Western plains.
Again, the surface of the plant may become covered with a felt of fine hairs to prevent rapid evaporation, while other plants with ordinary foliage have the acquired power of moving the leaves so that they will expose their surfaces broadside to the sun, or contrariwise the edges only, as heat and light intensity determine.
Phytoecology deals with all those adaptations of structure, and from which permit the plants to take advantage of the habits and wants of animals. If we are studying the vegetation of a bog, and note the adaptation of the hydrophytic plants, the chances are that attention will soon be called to colorations and structures that indicate a more complete and far-reaching adjustment than simply to the conditions of the wet, spongy bog. A plant may be met with having the leaves in the form of flasks or pitchers, and more or less filled with water. These strange leaves are conspicuously purplish, and this adds to their attractiveness. The upper portion may be variegated, resembling a flower and for the same purpose—namely, to attract insects that find within the pitchers a food which is sought at the risk of life. Many of the entrapped creatures never escape, and yield up their life for the support of that of the captor. Again, the mossy bog may glisten in the sun, and thousands of sundew plants with their pink leaves are growing upon the surface. Each leaf is covered with adhesive stalked glands, and insects lured to and caught by them are devoured by this insectivorous vegetation.
In the pools in the same lowland there may be an abundance of the bladderwort, a floating plant with flowers upon long stalks that raise them into the air and sunshine. With the leaves reduced to a mere framework that bears innumerable bladders, water animals of small size are captured in vast numbers and provide a large part of the nourishment required by the highly specialized hydrophyte.
These are but everyday instances of adaptation between plants and animals for the purpose of nutrition, the adjustment of form being more particularly upon the Vegetative side. Zoölogists may be able to show, however, that certain species of animals are adapted to and quite dependent upon the carnivorous plants.
An ecological problem has been worked out along the above line to a larger extent than generally supposed. If we should take the case of ants only in their relation to structural adaptations for them in plants, it would be seen that fully three thousand species of the latter make use of ants for purposes of protection. The large fighting ants of the tropics, when provided with nectar, food,