The oat of commerce is a typical grass, and from a study of its parts one can gain knowledge on the structure of grasses in general.
To a casual glance there is little difference between the oat-blossom and the grain ready for the harvest. The flower-cluster is green. The ripe oat-cluster or "fruit" is yellow. The non-botanist would find no other distinction between flower and fruit. Indeed, he probably would not recognize the flower as a flower nor the "fruit" as a fruit.
What looks like one grain in the oat-cluster is - little as one might think so - two flowers, and between them there is generally a little white affair, which is the last vestige of a third (Fig. 40, a).
The whole trio constitute a "spikelet." Most grass-flowers grow thus in spikelets, which are little floral households.
Outside the oat-spikelet there are two chaffy pointed green scales.
These are the "outer" or "empty" glumes. They correspond to the involucre, or circle of little green scales which surrounds the whole head of bloom in many clustered flowers (Fig. 40, b). All grass spikelets are thus partly or wholly enclosed in one or two, or sometimes more than two empty glumes. Sometimes they are so small that Nature seems in fair way to abolish them altogether.
Fig. 40. - Oats and yarrow.
a, A cluster or "spikelet" of oat-blossoms; b, a cluster or "head" of yarrow-blossoms; c, a single oat-blossom with its enfolding "glumes"; d, a single yarrow-blossom with its attendant "bracteole".
Sometimes, as in the oat-blossom, they are large enough to shut the whole spikelet in between them.
When we separate the empty glumes of the oat-spikelet we find the enclosed flowers (Fig. 40, c). Each is shut in between two green scales, which are like the "empty glumes," but smaller, and blossom and scales together look like a green oat-grain.
Scales similar to these enclose the blossoms of all typical grasses. They are generally two in number, and are sometimes called "flowering glumes" and sometimes "paleae," while Gray's "Manual" calls the lower and outer one the "flowering glume" and the upper and inner one the palet. However designated, they correspond to those scale-like leaves which stand beside the florets of many-clustered flowers, and are variously named by the technical botanist (Fig. 40, d).
At some fair future day we will have, let us hope, the same name for the same organ, whatever the organism in which it occurs. This plan will save the nature-student much tribulation, and will give him far clearer ideas of relationship and development than he can possibly get under the present system.
Meantime we call these little green affairs "bracteoles" when they stand beside the florets of the yarrow and "flowering glumes" and "paleae" when they enclose the tiny blossoms of the grasses.
But they occupy a similar post in the plant economy everywhere. They are humble attendants upon the true flowers, standing close by as if to guard and screen them.
We shall see the oat-blossom itself when the flowering glume has been removed. Its most conspicuous parts are a pistil and three stamens. The anthers are large, as in all the grasses, and they are balanced like see-saw boards, on the tips of slender filaments.
So they oscillate and sway at the faintest breath, shaking their pollen out to the wind. The filaments, which are as fine as gossamer, are also stirred by the faintest zephyr.
And, lastly, the spikelet itself dangles at the end of a delicate stalk, which forms part of an open, swaying flower-cluster. So the wind has its will with the oat-blossoms, and its force is used to the utmost in shaking the stamens and scattering the pollen. The pollen is light and dry, so that it can readily be detached from the anthers, and blown away:
And the pistil is especially fitted to catch the precious dust as it flies.
The stigma is proportionately long and large, and forks into two parts.
These spread widely asunder, as if welcoming the pollen with open arms; and they are hairy and somewhat glutinous, so that the golden grains which come to them may catch and cling.
But in the anatomy of grasses and of oats, among the rest we find hints that the cooperation between them and the wind has not always been so perfect as it is to-day.
For the flowers still have vestiges of petals, and hence we surmise that once upon a time they lured insects, and were fertilized by them.
When the wind became the pollen carrier for the grass-blossoms, their petals were no longer needed as insect lures. So they grew "small by degrees and beautifully less".
Some grasses have three of these moementoes of bygone glories, others have only two (Fig. 41). They are minute affairs, transparent or translucent, and very pretty under a low-power microscope even in their present degradation. When the stamens and pistil are matured these reminiscences of petals become succulent, and thus force palet and flowering glume apart, so that the stamens can dangle out to the wind, and the pistil can reach abroad for pollen.
And by thus making themselves useful in a new capacity the superseded petals have saved their lives.
Had they been less versatile they might have shared the fate of some sedge petals, which have shrivelled and shrunk to the vanishing point.
When the pistil has been fertilized, the flowering glume and palet close together again and form a protective covering for the ripening fruit.
The ovary has but one ovule, so this fruit contains but one seed. It is wrapped in two coats, as seeds generally are, and outside these are three more coats, which constitute the envelopes of the fruit.