THE ANGIOSPERMAE 



1575 



absorptive tip is often swollen and tuberous, or else is drawn out into an 

 attachment clearly differentiated from the leaf-like portion of the coty- 

 ledon, as in Tradescantia, Crocus and many other types (Fig. 1435). It thus 

 forms a distinct organ of absorption or haiistorium. This differentiation of 

 a suctorial portion of the cotyledon is important in the interpretation of the 

 Grass embryo, as we shall see later. 



Fig. 1435. — Tradescantia virginica. A, Seedline;. The 

 coleoptile forms the apex, showing the slit from 

 which the first leaf emerges. The tip of the coty- 

 ledon is still embedded in the seed, while the 

 " middle piece " of the cotyledon is joined to 

 the base of the coleoptile and partly fused to the 

 hvpocotyl. B, Embryo extracted from the seed. 

 The cotyledon is on top. C, Early stage in germin- 

 ation showing the haustorial tip of the cotyledon 

 in the endosperm. {After Goebel.) 



The lateral scutellum in Gramineae acts as a haustorium (Fig. 1436). 

 Its dorsal surface abuts on the endosperm and during germination the cells 

 of its protoderm elongate and separate from each other, closely resembling 

 root hairs. Like the latter they have very thin walls and are rich in proto- 

 plasm. When the endosperm is exhausted, these absorptive cells collapse. 



The most remarkable haustoria are formed in the seeds of the Palms. 

 In principle they are similar to the others, being specialized portions of the 

 cotyledon, but they reach an immense development. In Phoenix dactylifera, 

 the Date Palm, the resting embryo is very small and is laterally placed in 

 the hard endosperm (Fig. 1437). At germination the tip of the cotyledon 



