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, blossoming 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 summit of the stem. (Another species much like it, but having several brownish flowers, is shown in Figure 115, _L_.) If this plant can be had, the structure of the ovule and embryo sac may be easily studied, by simply stripping away the tissue bearing the numerous minute ovules, and mounting a few of them in water, or water to which a little sugar has been added. [Illustration: FIG. 79.--_A_, ripe ovule of _Monotropa uniflora_, in optical section, × 100. _m_, micropyle. _e_, embryo sac. _B_, the embryo sac, × 300. At the top is the egg apparatus, consisting of the two synergidæ (_s_), and the egg cell (_o_). In the centre is the "endosperm nucleus" (_k_). At the bottom, the "antipodal cells" (_g_).] 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 some time, we shall find in the centre of the embryo sac a large nucleus (_k_) (or possibly two which afterward unite into one), and at each end three cells. Those at the base (_g_) probably represent the prothallium, and those at the upper end a very rudimentary archegonium, here generally called the "egg apparatus." Of the three cells of the "egg apparatus" the lower (_o_) one is the egg cell; the others are called "synergidæ." The structure of the embryo sac and ovules is quite constant among the angiosperms, the differences being mainly in the shape of the ovules, and the degree to which its coverings or integuments are developed. The pollen spores of many angiosperms will germinate very easily in a solution of common sugar 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 warm weather. The spores may be sown in a little of the sugar solution in any convenient vessel, or in a hanging drop suspended in a moist chamber, as described for germinating the spores of the slime moulds. The tube begins 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 some other forms (Fig. 79). [Illustration: FIG. 80.--Germinating pollen spores of the sweet pea, × 200.]