DICOTYLEDONES 



401 



reaching the ovary, penetrates the ovule from the base (chalaza) 

 instead of entering the micropyle. It is not probable, however, that 

 this can be considered to be of great importance in classification, as 

 chalazogamy has been observed in several genera, which are probably 

 not closely related. The fusion of the second generative nucleus 

 with the endosperm-nucleus has been observed in a number of cases, 

 but is probably not universal. 



The Embryo 



The typical dicotyledonous embryo (Fig. 377) differs from that of 

 the Monocotyledons in having the apex of the stem terminal, with 

 two opposite cotyledons of equal size. Sometimes e.g. Peperomia, 



FIG. 377. A-D, Capsella bursa-pastoris, development of the embryo; I, optical 

 sections (X200). E, F, Senecio aureus, young embryos, longitudinal sections 

 (X270); sus, suspensor ; b, primary suspensor-cell ; cot, cotyledons ; system-apex; 

 r, root. (E, F, after MOTTIER.) 



Monotropa the embryo in the ripe seed (Fig. 378) is very small and 

 its members undeveloped. More commonly the organs are well 

 developed, and, besides the cotyledons, the terminal bud (Plumule) 

 may be well developed. The stem (Hypocotyl) and root (Radicle) 

 can usually be distinguished. The embryo may be surrounded by 

 abundant endosperm (Convolvulus, Celastrus, Ricinus, etc.), or the 

 thick, fleshy cotyledons may completely fill the embryo-sac (Fagaceae, 

 Leguminosse). In a few instances (Nympheeacese, Piperacese) peri- 

 sperm largely replaces the endosperm. 



A single cotyledon has been reported for Nelumbo, and it is prob- 

 able that the apparently coherent cotyledons of Podophyllum may 

 represent a single one. Both of these plants show other monocoty- 



2D 



