556 



LIFE-HISTORIES 



Fig. 382. — Climbing Buckwheat (Polygonutii Convolvulus, Buckwheat 

 Family, Polygonaceae). Pistil (V) during fertilization, cut vertically 

 to show stalk-like base (/s) of ovary; stalk of ovule {fu)\ end of stalk 

 {cha) ; the nucellus {nu), the micropyle, or opening to the nucellus {mi); 

 the inner integument (n); the outer integument {ic); the embryo-sac 

 (c) ; nucleus of the embryo-sac {ek) ; the egg-apparatus {ei) consisting 

 of three cells, the lower one being the egg-cell and the two others com- 

 panion cells which are thought to represent rudimentary archegonia; 

 prothallial cells {an); style {g); stigma («.); pollen grains {p); and 

 pollen-tubes {ps). A pollen grain falling upon the stigma produces a 

 tube which grows down through the style, enters the micropyle, and 

 penetrates to the egg-cell; here it discharges its (male) nucleus which 

 fuses with the nucleus of the egg-cell and from this union an embryo 

 arises. (Strasburger.) — The plant is an annual vine resembling buck- 

 wheat but with greenish flo^vers; native to Europe but a common 

 weed in America. 



Fig. 383.— Shepherd's Purse {Capsella Bursn-pastoris, Mustard Family, 

 Criicifercp). Development of the embryo. A-D. successive stages, much 

 magnified, showing the suspensor {ct), lower end of embryo {h), cotyle- 

 dons (c), and the stem-tip (p) from which the plumule arises. (Han- 

 stein.)— The plant is annual, about 50 cm. tall, with small white flowers 

 and dry fruit; native of Europe; common as a weed in America. 



