DICOTYLEDONS. 



is diclinous, even only of a single ovary or of a single or several stamens; the 

 perianth being either entirely absent (as in Salix and Piperacecfi) or reduced to a 

 cup-like structure (Populus, the female flower of Cannabinece &c.) or to hair-like 



FIG. 433.-Diag:raiu of PapaveraceK ; A Chdidonium, a Papaver. 



scales among the sexual organs which represent the flower {e.g. Platanus). Flowers 

 of this kind are generally very small and densely crowded in large numbers in 

 the inflorescence (such as capitula, spikes, or catkins). In some cases it may even 

 be doubtful whether we have an inflorescence or a single flower, as in the genus 

 Euphorbia \ 



The development of the separate parts and the entire form of the flower 

 in the mature state is so various that it is scarcely possible to state any general 

 facts concerning them. The perigynous structure of the flower is peculiar to 

 Dicotyledons, as is also the occurrence of hollowed axes of the inflorescence, like 

 the fig and similar structures, and the cupule, which occur in some families, and are 

 dependent on similar processes of growth. 



The Ovules exhibit, in the difl"erent divisions of Dicotyledons, all those varieties 

 of structure which have already been mentioned in the introduction. Very commonly, 

 especially among the Gamopetaloe, the nucleus is covered by only one integument, 

 which is then often very thick before impregnation. But on the other hand the 

 third integument or aril is much more common than among Monocotyledons. 

 When there are two integuments, the outer one, — diff"ering again in this respect 

 from most Monocotyledons — takes part in the formation of the micropyle, enveloping 

 the exostome or entrance to it. In some parasites the ovules are rudimentary, 

 and in many Balanophoraceae are reduced to a naked few-celled nucleus; while 

 in Loranthaceae they are coherent with the tissue of the floral axis in the inferior 

 ovary. 



The behaviour of the Embryo-sac'^ before and after impregnation is similar 

 in most Dicotyledons to that which occurs in Monocotyledons. The endosperm 

 usually originates by free cell-formation, and is transformed by repeated divisions 

 of the first cells which are formed in this manner into a more or less dense tissue, 

 which fills up the embryo-sac either before or after the formation of the multi- 

 cellular rudiment of the embryo. But in a very considerable number of families 

 belonging to altogether different groups the embryo-sac exhibits on the one hand 

 striking phenomena of growth, elongating considerably before impregnation into 



^ See Payer, /. c. p. 529 ; [also foot-note to p. 426]. 



2 Hofmeister, Jahrb. fur wiss. Bot. vol. I, p. 1S5 ; and Abhandl. der kon. Sachs. Ges. der Wiss. 

 vol. VI, p. 536. 



