CHAPTER XII 



ANGIOSPERMvE (Continued.) 

 SUBCLASS II. DICOTYLEDONES 



THE Dicotyledons are preeminently the modern plant type, far 

 outnumbering the Monocotyledons both in the number of species 

 and of individuals. They also show much greater range of struc- 

 ture. The lower Dicotyledons resemble to some extent the simpler 

 Monocotyledons, to which they are undoubtedly related ; but there 

 is still some question as to which group is the older, and it is pos- 

 sible that the two groups may have originated independently from 

 a common stock. 



The Gametophyte 



The male gametophyte of the Dicotyledons does not differ from 

 that of the Monocotyledons. The small antheridial cell divides into 

 two generative cells, and may occasionally (Caltha, Silphium, Pepe- 

 romia) assume a coiled form suggestive of the spermatozoid of the 

 Archegoniates. The germination of the pollen-spore, and develop- 

 ment of the pollen-tube, is of the usual angiospermous type. 



The female gametophyte is usually of the typical form, but there 

 are some exceptions. Thus, in Peperomia, there are normally six- 

 teen nuclei in the unfertilized embryo-sac, and no definite egg- 

 apparatus or antipodal cells are present. In Casuarina it is possible 

 that a similar condition exists, but this is still doubtful. In some 

 other low types among Dicotyledons, e.g. Santalacese, Phoradendron, 

 the ovules are imperfectly developed, the embryo-sac arising from 

 the tissue of the placenta. These latter forms are mostly parasites, 

 and it is likely that their peculiarities are secondary acquisitions. 



The antipodal cells are usually three, but the number is not in- 

 frequently greater (many Compositse), or the nuclei of the large 

 antipodals may divide, as in some Ranunculacese. 



Fertilization 



The lower Dicotyledons are often wind-pollinated, but as a rule 

 they are entomophilous. In a number of cases that have been 

 examined (e.g. Casuariua, Juglans, Betula, etc.) the pollen-tube, after 



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