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A TEXTBOOK OF THEORETICAL BOTANY 



cereals. An analogous cambium forms the protecting suberized layer 

 around the endosperm of the naked seed of Criniim, already mentioned. 

 Where the whole of the reserve material is contained in the tissues of 



the embryo, germination is generally 

 quicker than in seeds with endosperm, 

 probably because the food substances 

 are more directly available. Where 

 the embryo is entirely surrounded by 

 the endosperm, the surface layer of 

 the embryo acts primarily as an ab- 

 sorptive layer and is only later differ- 

 entiated into an epidermis or a pili- 

 ferous layer, that is to say it retains the 

 character of embryonic protoderm 

 until the endosperm is exhausted. The 

 lateral or curved embryos in some 

 seeds may only be in contact with the 

 endosperm at certain places, for ex- 

 ample at the abaxial side of one cotyle- 

 don. The protoderm retains its un- 

 differentiated nature only at these 

 places, whereas the other cotyledonary 

 surfaces develop epidermis and stomata 

 immediately. Later on the absorptive 

 surface also differentiates into typical 

 epidermis. 



Specialized absorbing organs are 

 found in some Monocotyledons. A 

 simple form is shown by seedlings of 

 the Allium type, where the cotyledon 

 is withdrawn from the seed coat dur- 

 ing germination, and becomes a green 

 leaf, but the tip remains embedded in 

 the endosperm within the seed and 

 continues the absorption of food sub- 

 stances until it finally withers and falls 

 off' (Fig. 1434). The protoderm in 

 these absorptive parts never becomes 

 permanently differentiated, and the 



Fk;. 1434. — Allium cepa. Onion. Ger- 

 mination. A, Seedling with the tip of 

 the cotyledon embedded in the endo- 

 sperm in the seed. B, Section of the 

 seed before germination, showing the 

 coiled embryo. C, The embryo freed 

 froni the seed. D, Emergence of the 

 radicle as the first stage of germina- 

 tion. (After Sachs.) 



