SEEDS AND SEEDLINGS 



137 



127. Classification according to number of cotyledons. The 



seeds of one great division of seed plants, the monocotyledons, 

 — comprising grasses, sedges, palms, lilies, 

 and many other groups, — have one cotyle- 

 don (Fig. 127). The reserve food is, as is 

 shown in that figure, mainly stored outside 

 the embryo. 



The seeds of the other and still larger 

 division, the dicotyledons, haA'e two cotyle- 

 dons (Figs. 125 and 126). The plant food in 

 the seeds of dicotyledons is often stored in 

 the embryo itself (Fig. 126), as in the chest- 

 nut, hazel, beech, 

 oak, bean, and sun- 

 flower; or often, 

 like that of the 

 monocotyledonous 

 onion(Fig.l27,^), 

 outside of the em- 

 bryo, as in buck- 

 wheat, four-o'clock, 

 castor bean, honey 

 locust, and morn- 

 ing-glory. 



128. Forms of 

 reserve material. 

 The study of the 

 forms of the food 

 stored in seeds is 

 in many ways 

 most important. 

 For a time, usu- 

 ally, the seedling 

 plant depends for 

 its growth largely on the reserves in the seed from \^"hich it 

 springs. And the most concentrated vegetable food used by 



Fig. 127. Seed and seedlings of onion 



A, seed ; B-F, successive stages in development of the 



seedling ; e, cotyledon ; e, endosperm ; /, first true leaf ; 



A, liypocotyl ; s, slit from which /emerges; r-i, primary 



root ; n, secondary root. A, considerably magnified 



