Part 14 (2/2)

The angiospere of plants, all those ordinarily called ”flowering plants” belonging here There is almost infinite variety shown in the for particularly the case with the flowers As already stated, the ovules, instead of being borne on open carpels, are enclosed in a cavity forle closed carpel or several united carpels To the organ so formed the name ”pistil” is usually applied, and this is known as ”simple” or ”compound,” as it is co the pollen spores are also much modified, and for leaves there are usually otheroften brilliantly colored and rendering the flower very conspicuous To these leaves surrounding the sporophylls, the general naone has a twofold purpose, serving both to protect the sporophylls, and, at least in bright-colored flowers, to attract insects which, as we shall see, are i pollen from one flower to another

When we coiosperreat difference is noticed, there being her pteridophytes Unfortunately there are very few plants where the structure of the embryo sac can be readily seen without very skilful manipulation

There are, however, a few plants in which the ovules are very small and transparent, so that they may be mounted whole and examined alive The best plant for this purpose is probably the ”Indian pipe”

or ”ghost flower,” a curious plant growing in rich woods, blosso in late summer It is a parasite or saprophyte, and entirely destitute of chlorophyll, being pure white throughout It bears a single nodding flower at the su several brownish flowers, is shown in Figure 115, _L_)

If this plant can be had, the structure of the ovule and e away the tissue bearing the nu a few of thear has been added

[Illustration: FIG 79--_A_, ripe ovule of _Monotropa uniflora_, in optical section, 100 _m_, micropyle _e_, e apparatus, consisting of the two synergidae (_s_), and the egg cell (_o_) In the centre is the ”endosper_)]

The ovules are attached to a stalk, and each consists of about two layers of colorless cells enclosing a central, large, oblong cell (Fig 79, _A_, _E_), the embryo sac or macrospore If the ovule is from a flower that has been open for soe nucleus (_k_) (or possibly thich afterward unite into one), and at each end three cells Those at the base (_g_) probably represent the prothalliuoniu apparatus”

Of the three cells of the ”egg apparatus” the lower (_o_) one is the egg cell; the others are called ”synergidae” The structure of the eiosper ree to which its coverings or integuiosperar in water: about fifteen per cent of sugar is the best A very good plant for this purpose is the sweet pea, whose pollen germinates very rapidly, especially in eather

The spores ar solution in any convenient vessel, or in a hanging drop suspended in athe spores of the sliins to develop within a few minutes after the spores are placed in the solution, and within an hour or so will have reached a considerable length Each spore has two nuclei, but they are less evident here than in so 79)

[Illustration: FIG 80--Ger pollen spores of the sweet pea, 200]

The upper part of the pistil is variouslyeither little papillae which hold the pollen spores, or are viscid In either case the spores gerh the tissues of the pistil until they reach the ovules, which are fertilized ymnosperms

The effect of fertilization extends beyond the ovule, the ovary and often other parts of the flower being affected, enlarging and often beco the various fruits of the angiosperms These fruits when ripe rains of various kinds, beans, peas, etc; or the ripe fruitin this way to attract animals of many kinds which feed on the juicy pulp, and leave the hard seeds uninjured, thus helping to distribute them Common examples of these fleshy fruits are offered by the berries of many plants; apples, melons, cherries, etc, are also faood deal both in regard to size and the degree to which the embryo is developed at the time the seed ripens

CLassIFICATION OF THE ANGIOSPERMS

The angiosperms are divided into two sub-classes: I _Monocotyledons_ and II _Dicotyledons_

The monocotyledons comprise many familiar plants, both ornaated, sed leaves with parallel veins, and the parts of the flower are in threes in the majority of them As their name indicates, there is but one cotyledon or seed leaf, and the leaves from the first are alternate As a rule the embryo is very small and surrounded by abundant endosperhly typical members of the sub-class are the lilies and their relatives The one selected for special study here, the yellow adder-tongue, is very co; but if not accessible, alarden flowers, the tulip, hyacinth, narcissus, or one of the common lilies may be used; of wild flowers, the various species of _Trilliu 83, _A_) are common and easily studied forms, but the leaves are not of the type coue (_Erythroniu 81) is one of the co in the northern states from about the middle of April till the middle of May

Most of the plants found will not be in flower, and these send up but a single, oblong, pointed leaf The flowering plant has two sier than the other They seeround, but closer examination shows that they are attached to a steround This arises from a small bulb (_B_) to whose base nu fro a single, nodding flower at the top

The leaves are perfectly sreen with purplish blotches above The epidermis may be very easily reitudinal rows of whitish spotspores

[Illustration: FIG 81--_A_, plant of the yellow adder-tongue (_Erythronium americanum_), ? _B_, the bulb of the same,_r_, roots _C_, section of _B_ _st_ the base of the ste the bulb for next year (_b_) at its base _D_, a single petal and staynciurown uments _sp_ macrospore (embryo sac) _H_, cross-section of the ripe anther, 12 _I_, a single pollen spore, 150, showing the two nuclei (_n_, _n'_) _J_, a ripe seed, 2 _K_, the saitudinal section _em_ the embryo _L_, cross-section of the steram of the flower]