SEXUAL REPRODUCTION. 



219 



In seed plants the embryo, which forms within an ovule (see 

 If 280), and stimulates it to renewed growth, develops to a 

 certain stage and then ceases to grow. With suitable protec- 

 tion and food supply, it is then cast 

 off as a seed, and, usually after a 

 dormant period, continues its de- 

 velopment until mature. The ripe 

 seed consists of the following parts: 

 (i) In the interior, occupying 

 various positions and of exceeding- 

 ly variable relative size, is the 

 embryo. (2) Immediately around 

 it lies a tissue containing reserve 

 food, but this may be so shrunken 

 and emptied as to be recognizable only by microscopic 

 examination. In that case the reserve food will have been 

 absorbed by the embryo itself, which is then likely to be 

 large and to occupy most or all of the space 

 within the seed coats. (3), Upon the exterior 

 one or two seed coats, more or less readily distin- 

 guishable from each other (figs. 185, 186). 



Fig. 185. — Seed of pansy, entire and 

 halved, the latter showing the 

 straight embryo, the endosperm 

 (white and dotted), the seed-coats ; 

 wz, micropyle. Magnified about lo 

 diam.— After Baillon. 



Induced result of cell union. 



306. Fruit. — The growth of the embryo 



Fig. 186. — Seed - 

 of pokeberry 

 {^Phytolacca 

 deca ndr a), 

 halved ; show- 

 ing curved em- excites not Only the ovule to further develoDT 



bryo next the 



two seed-coats meiit, but also the carpels which bear the ovules, 



and nearly sur- ' '■ 



rounding the and not infrequently even more remote parts. 



endosT3erm. ^ •' ^ 



, Magnified The carpels and their contents and adherent 



about 10 diam. ^ 



— AfterBaiiion. parts, when fully developed, constitute \h& fruit. 

 The carpels are then known as the pericarp. The changes 

 which the parts undergo are chiefly of two sorts — an increase 

 in size and an alteration of texture. The increase in size re- 

 quires no special explanation. The carpels may become 

 dry at maturity, or may thicken and become soft and fleshy, 



