648 PHANEROGAMS. 



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 diff'erent divisions of Dicotyledons, all those varieties 

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

 especially among the Gamopetalae, the nucellus is covered by only one integument, 

 which is then often very thick before fertilisation. 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 — difl'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 Balanophoraceas are reduced to a naked few-celled nucellus; 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 fertilisation 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 diff'erent groups the embryo-sac exhibits on the one hand 

 striking phenomena of growth, elongating considerably before impregnation into 

 a long tube, and emitting after impregnation one or more vermiform protrusions 

 which penetrate into and destroy the tissue of the nucellus and of the integu- 

 ments, or even protrude altogether out of the ovule (as in Pedicularis, LathrcBa^ 

 and Thesiuui). On the other hand, in those plants in which the endosperm 

 originates by cell-division we learn from Hofmeister that the following variations 

 occur: — 'The whole of ihe cavity of the embryo-sac behaves like the first cell 

 of the endosperm in Asarineae, Aristolochiaceae, Balanophoracese, Pyroleae, and 

 Monotropeae ; the first division of the sac is the result of a partition-wall which 

 divides it into two nearly equal halves, each of which encloses a cell-nucleus 

 and again divides at least once into daughter-cells. In other cases the first 

 cell of the endosperm includes the upper end of the embryo-sac ; the embryo-sac, 

 immediately after fertilisation, appears to be divided by a transverse septum into 

 two halves, the upper one of which developes into the endosperm by a series of 

 bipartitions \ while no such bipartition of the lower one occurs in Vtscum, Thestutfi, 

 Lathrcea, Rhmanlhus, Afazus, Melampyrum., or Globularia. The first cell of the 

 endosperm fills up the middle part of the embryo-sac in Veronica., Nemophila, 

 Pedicularis, Plantago, Campanula, Loasa, and Labiatae ; its lower end in Loranlhus, 

 Acanthus, Catalpa, Hebenstreitia, Verbena, and Vaccinium.'' In NymphcEa, Nuphar, 

 and Ceratophyllum, the upper end of the embryo-sac is cut off" from the rest 

 of the space by a septum soon after fertilisation, and the further development of 



^ Hofmeister, Jahrb. fiir wiss. Bot. vol. I. p. 185 ; and Abhandl. der kon. Sachs. Ges, der Wiss. 

 vol. VI. p. f^ > ^i;ee supra, p. 579). 



