1592 



A TEXTBOOK OF THEORETICAL BOTANY 



[Triglochin) and many Liliaceae are of almost equal simplicity, though the 

 hypocotyl is sometimes lacking and the tip of the cotyledon remains for 



Fig. 1 45 1. — Naios major. A, Seedling whole and in section. Alisma plaiitago- 

 aqiiatica. B to E, Stages of germination, the cot\'ledon being simple, 

 erect and epigeal. {After Velenovsky.) 



some time in the endosperm, the cotyledon being, however, erect and 

 carrying the seed up with it. The evolution of the haustorium can be traced 

 through a series of types (Fig. 1452), beginning with forms like Allium, 

 where it is scarcely differentiated and is simply the deflexed apex of the 

 cotyledon, passing to a stage like that in Iris where the haustorium in the 

 seed is united to the apex of the cotyledon by an elongated middle piece; 

 next to a stage where the middle piece is laterally attached to the cotyledon 

 {Crocus) and finally, as in Tradescantia, to the attachment of the middle piece 

 to the base of the cotyledon. This leads us to the embryo of the Gramineae, 

 the most highly specialized and the most difficult to interpret. 



Its principal characters are shown in the section of the embryo of Zea 

 mais, Fig. 1453. 



They may be briefly enumerated as follows: the haustorial scutellum, 

 embedded in the caryopsis and frequently bearing a small ventral scale 

 attached to its upper portion; the epiblast, a small scale opposite the 

 scutellum; the coleoptile, a sheath, sometimes colourless, enclosing the well- 

 developed plumular leaves; the coleorhisa, a sheath surrounding the rudi- 

 rnent of the primary root. 



