144 



BOTANY. 



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 struct- 

 ure of the ovule and embryo sac 

 may be easily studied, by simply 

 stripping away the tissue bearing the 

 numerous minute ovules, and mount- 

 ing a few of them in water, or water 

 to which a little sugar has been 

 added. 



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, JE"), 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 arche- 

 gonium, here generally called the 

 "egg apparatus." 



Of the three cells of the "egg ap- 

 paratus " the lower (o) one is the egg 



cell ; the others are called " synergidse." 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 



FIG. 79. A, ripe ovule of Mono- 

 tropa uniflora, in optical section, 

 x 100. m, micropyle. e, embryo 

 sac. B, the embryo sac, x 300. 

 At the top is the egg apparatus, 

 consisting of the two synergidae 

 (s), and the egg cell (o). In the 

 centre is the " endosperm nucleus " 

 (fc). At the bottom, the "anti- 

 pedal cells " (g). 



