372 



BOTANY 



Less frequently the stem-apex arises from the terminal segment of the young 

 embryo, and the single cotyledon is borne at its side. This occurs in Sparganium 



and Zannichellia (Fig. 



A B 



cot 



Rt 



341), and has also 

 been described for 

 the Dioscoreacese and 



V-CtU^iA /ll^ifrfy^ 8 ^^ some other Monocot- 



yledons. The em- 

 bryo in these forms is 

 intermediate in char- 

 acter between the 

 typical Monocotyle- 

 dons and the Dicoty- 

 ledons. 



Sometimes a sus- 

 pensor is quite want- 

 ing (some Aracese 

 and Graminese), and 

 there is a suggestion 

 of the regular quad- 

 rant divisions found 

 in the Pteridophytes. 



The absence of a suspensor is associated with the early investment of the embryo 



by the endosperm-cells. 



FIG. 341. A, Zannichellia palustris, section of embryo 

 (X250); cot, cotyledon; st, stem-apex. B, Lysichiton 

 Kamchatcense, the embryo does not develop a suspensor, 

 and is early surrounded by the endosperm, end (x 250). 



The Endosperm 



The primary endosperm -nucleus always divides, and usually gives rise to many 

 secondary nuclei before any cell- walls appear. Where the embryo develops early, 

 as in Naias, the endosperm remains rudimentary, but in most Monocotyledons it 

 is largely developed. In Naias flexilis the endosperm is formed from the upper 

 one only of the two nuclei resulting from the division of the primary endosperm- 

 nacleus. The lower one remains undivided, but increases very much in size. 

 The endosperm is usually formed by free-cell formation, that is, by the simul- 

 taneous formation of cell-walls between the free nuclei, and the formation of 

 cellular tissue proceeds from the periphery toward the centre of the sac (Fig. 

 322, B). In some Aracese, cell-walls extending across the cavity of the embryo- 

 sac are formed at an early stage, and the embryo-sac is from the first completely 

 filled with the prothallial tissue. 



Where the endosperm is present in the ripe seed, its cells are filled 

 with starch, oil, or other nutritive substances. In other cases e.g. 

 many Palms (Date, Phytelephas) the reserve food is in the form 

 of cellulose, developed in the greatly thickened walls of the endo- 

 sp arm-cells. 



Germination 



The cotyledon may become a foliage-leaf (Onion) (Figs. 342-344) ; 

 but more commonly, as in the Grasses and Palms, the cotyledon re- 

 mains permanently within the seed, acting as an organ for the ab- 

 sorption of the food-materials in the endosperm. By the downward 



