Very carefully Davy took away the ring of stamens, and left only the little yellowish-green center of the sorrel flower. "This is where we get the seed," he said, as gravely as an old college professor lecturing to a class. "This is the pistil, and it has three parts, too: the pod, the style, and the stigma. The stigma is the little piece at the top which catches the pollen from the anthers. The style is the stem, and the pod is the big part below which holds the seeds." He held up the little stripped flower again. "This pistil has five styles and five stigmas," he went on. "A good many flowers have more than one. It has ten stamens, too--two stamens for each style, and five petals and five sepals. You can divide it by five all the way through." "Even to the seed-pod," added the Chief Gardener. "It has five divisions," and he cut the tiny green pulp and showed them with his magnifying-glass. "The little sorrel flower is one of the most perfect of flowers--one of the most perfect in a great class of flowers called Ex-o-gens. There is one other class called End-o-gens. Those words are from the Greek, too. Exogen means outward-growing. Endogen means inward-growing. The stem of an Exogen grows by layers, as most trees grow." "Oh, yes," said Prue, "I know. We counted the rings on that big oak that was cut down over by the lake last year. It had one ring for each year." [Illustration: A GROUP OF ENDOGENS--THE LILY, HYACINTH, AND DAFFODIL] "That's right, Prue, and the stem of the Endogen grows inside a shell, and is often just a soft pith, like the inside of a cornstalk. These are the two great classes of all flowering plants and trees. You can always tell the difference by their stems; nearly always from their leaves; always from their seeds, if you have a strong magnifying-glass, for the little germ of the Exogen has two leaves like the morning-glory, and the germ of the Endogen has but one like the lily, or corn. But the easiest way for you to tell is by the flowers. An Exogen flower nearly always goes by fives, like the little sorrel bloom, sometimes by fours, but hardly ever by threes. The Endogen flower is nearly always divided in threes, like the lily, which has six petals. It very seldom has four parts, and never five. So, you see, we know right away that the sorrel and the rose and buttercup are Exogens, and that the lily and the hyacinth and the daffodil are Endogens. Of course, there are many flowers not so easy to place as these, and I am afraid I am giving you too hard a lesson for one time, especially for such a hot day."