1502 



A TEXTBOOK OF THEORETICAL BOTANY 



While the cotyledons are the usual repositories of nutriment in the 

 absence of endosperm, the hypocotyl sometimes takes their place. For 

 example, in BerthoUetia excelsa, the Brazil nut, and in some Guttiferae, 

 the seed is filled by the immensely enlarged hypocotyl of the embryo (Fig. 

 1376), the cotyledons being scarcely visible as two minute, overlapping 

 scales at one end and the radicle forming a mere point at the other. Indeed, 



Fig. 1376. — Beitholletia excelsa. A, L-ongitu- 

 dinal section of seed which is filled by the 

 enlarged hypocotyl. The minute coty- 

 ledons are shown at the top. B, Enlarge- 

 ment of the folded cotyledons. C, Surface 

 view of the cotyledons. (After Goebel.) 



in Garcinia the radicle is so reduced that the primary root soon aborts and is 

 replaced by an adventitious root from the hypocotyl after germination. 

 Similarly, in Zostera, Riippia and some allied genera, the hypocotyl is 

 enlarged into a big rectangular body, from the side of which the single 

 cotyledon protrudes like a horn. 



In Monocotyledons the single cotyledon may show the same division 

 into blade and sheath which is characteristic of the foliage leaves. This is 

 always the case in non-endospermic monocotyledonous seeds, i.e., in Buto- 

 maceae and Alismaceae (Fig. 1377). These cotyledons emerge above 

 ground as assimilatory organs, while in most endospermic seeds the whole 

 cotyledon, or sometimes only its apex, is modified into a haustorium in 

 the endosperm, as we shall describe in more detail later (p. 1574)- In the 

 embryology of Monocotyledons it is clear that the cotyledon is formed by 

 the terminal segments and the hypocotyl and stem apex from the segments 

 below. This would appear to indicate that the cotyledon is a terminal 

 organ and the growing point lateral. The matter is, however, susceptible 

 to argument and we shall recur to the question of the relationship ot the 

 monocotyledonous and dicotyledonous types when we are dealing with 

 germination. 



Seeds may present a normal external appearance and yet contain no 

 embryo. This has been considered an uncommon occurrence, but recent 

 work has shown that it is widespread in the Umbelliferae. The endosperm 

 may be perfect but there is either no embryo or only an immature embryo 



