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MORPHOLOGY OF THE ANGIOSPERMS 



of both phloem and endarch xylem. The hypocotyl is not set off sharply 

 from root and stem; it is not an organ sui generis — though sometimes 

 so called. In some taxa, there is an external line of limitation — the 

 "collet" — between hypocotyl and root; the root is smaller in diameter 

 than the hypocotyl, and its epidermis is piliferous. (The term collet has 

 also been applied to the cotyledonary sheath, as well as to this collar- 

 like thickening of the hypocotyl. ) Internally, there are differences in 

 vascular structure. In most dicotyledons and some monocotyledons, the 

 hypocotyl is a prominent part of the axis; in other monocotyledons, the 

 hypocotyl is short and may be represented by hardly more than a vascu- 

 lar plate, which may be difficult to distinguish. The monocotyledons 



Fig. 124. Median longitudinal sections of embryos of grass genera in the Chlorideae, 

 showing variety of form within one tribe of the Gramincae. Epiblast (e) strongly 

 developed in E and F. A, Chloris verticillata; B, Schedonnardus paniculatus; C, 

 Bucldoe dachjloides; D, Lcptochloa scabra; E, Dactijloctenium aegypticum; F, 

 Eletisine indica. (After Reeder.) 



are sometimes described as having no hypocotyl, an inaccurate state- 

 ment. If both stem and root are present in the seedling, a transition 

 region must exist, even though it is platelike, a millimeter or less long — 

 a region perhaps detectable only with microtome sections. The hypo- 

 cotyl in monocotyledons is perhaps typically only a few millimeters in 

 length, but is long in some families, especially the Liliaceae and 

 Amaryllidaceae. 



The term transition region is applied to the part of the hypocotyl in 

 which the changes in vascular tissue occur; the hypocotyl may have 

 root structure below and stem structure above this region. The transi- 

 tion region may be low, that is, short; or high, that is, elongate, even 

 sometimes several centimeters long. A high transition region usually 

 accompanies elongate hypocotyls; a low transition region is usually 

 present in massive hypocotyls. 



