In a wild state, the air-plants grow on the bark of trees or on other substances, but they send their little roots into the moist bark or moss to get water. They do not feed on the juices of the trees, as parasites like the mistletoe do; they only want a standing-place, something to push against as they grow, and water. In the greenhouse they are usually planted in pots filled with bits of stone and damp moss, or they grow attached to the parent plant, as you may see in Fig. 2, and send their roots out into the air for food. A few of them—the Indian moccasin, for instance—grow like common plants in the ground.
It would almost seem as if the orchids had an eye to business in their imitation of insects. At any rate, there seems to be a very good understanding between them, and constant business relations are kept up. The flowers always have a little pouch somewhere about them in which they keep a stock of honey on hand. Their beautiful colors and delicious smell attract, by day and night, bees, butterflies, and moths. In return for the "treat" which the flowers give, the insects render a valuable service to the plants.
Fig. 3.—Honey Pouch and Pollen Pods.
I must remind you of something we have looked into before in "Picciola" (February 14, 1882), and that is that every perfect seed is the result of a partnership entered into by the pollen grains and the ovules of a flower. The pollen is the yellow dust which it is so easy to see on lilies and some other flowers; the ovules are little round bodies lying in the swollen part of a flower where it joins the stem. Above the ovules, and connected with them, is the pistil, sometimes standing up like the lily pistil of the geranium, sometimes only a sticky little pad, as it is in the orchids. Some plants get along perfectly well if this partnership is entirely a family affair, and the pollen of a flower falls on its own pistil, and makes a union with its own ovules, but this is not always the case. Certain plants require that the pollen shall be from another plant if the seed is to be sound and healthy. Orchids require this cross-fertilization, as it is called, and without the help of insects it could not be effected.
Bees and butterflies, it has been found out, always go in a single excursion from one flower of a kind to another of the same kind. They do not mix their drinks. This instinct not only serves to keep the honey stored by the bees pure, but it enables the insects to carry the pollen just where it will be useful. The pollen of a morning-glory would die if put on the rose pistil. It must be placed on a flower of the same family as the one it came from, or very nearly related to it, or it will do no good.
Fig. 4.—Pencil and Needle, with Pollen.
Now look at Fig. 2, and you will see that the flowers have a hollow tube in the centre, with a projecting lower lip. This tube is a single flower leaf curled over to make a tunnel, and through this tunnel is the only path to the honey pouch. When a butterfly feels like taking a drink, and one of these orchids is near, he lights on the lower lip of the tube, and pushing his long proboscis or trunk through it into the pouch, sucks up the honey. Now look at Fig. 3. This is a picture of the tube with its near wall cut away so that you can see the inside arrangement. As he works his proboscis down into the honey pouch, N, it is pressed against r, and touches a spring there; the little cap at r snaps open, and leaves a sticky ball resting on the proboscis. As the butterfly goes on sucking, this ball dries as if it were glued to his trunk. When he draws his head out, this proboscis is ornamented with one or two little tufts which look like the trees in a child's toy village, as you will see in the illustration.
