460 - FOURTH GROUP. — SEED-PLANTS. 



usually very small, and generally closely crowded in inflorescences with many flowers, 

 such as capitula, spikes and catkins. In some cases it may be a question whether we 

 have to do with an inflorescence or with a single flower, as in the genus Euphorbia^ . 



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

 whole flower in its developed state in the Dicotyledons is so various, that scarcely any 

 general statements can be made about them. Perigynous flowers are peculiar to 

 Dicotyledons, as is also the hollowed axis of the inflorescence in the fig and in 

 similar structures, as well as the cupule of some families, all of which are based on 

 similar processes of growth. 



The ovules show in the dilferent divisions of the Dicotyledons all the varieties of 

 structure which have been already mentioned in the introduction (p. 382); the 

 nucellus, especially in the Gamopetalae, has often but one integument, which is then 

 usually very thick before fertilisation ; on the other hand the third integument, the 

 aril, is much more common here than in the Monocotyledons ; if there are two 

 integuments, the outer one takes part in the formation of the micropyle, which is not 

 the case in most Monocotyledons, and surrounds the entrance to it, the exostome. The 

 ovules of some parasitic Dicotyledons are rudimentary ; in many of the Balanophoreae 

 they are reduced to a naked nucellus consisting of a few cells ; in the Loranthaceae, 

 where they spring from a free central placenta, they coalesce with the tissue of the 

 floral axis in the inferior ovary. 



The processes in the cmbiyo-sac- of most Dicotyledons before and after 

 fertilisation are similar to those in Monocotyledons ; the endosperm is usually 

 formed by free cell-formation, and developes by repeated division of the primary cells 

 thus formed into a more or less compact tissue, which fills the embryo-sac either at 

 an early period before the appearance of the pluricellular embryo, or some time after. 

 In a very considerable number of families belonging to quite different groups the 

 embryo-sac shows some striking phenomena of growth ; it may lengthen out before 

 fertilisation till it becomes only a narrow tube, and after fertilisation send out one or 

 many vermiform extensions which either penetrate laterally and with destructive effect 

 into the tissue of the nucellus and integuments, or even project free from the ovule 

 as in Pediciilaris, Lathraea, Thesium, etc. But the endosperm may also be formed by 

 division of the embryo- sac. In the latter case the following variations have been 

 observed by Hofmeister : ' the entire cavity of the embryo-sac serves as first cell of 

 the endosperm in the Asarineae, Aristolochiaceae, Balanophoreae, Pyroleae, and 

 Monotropeae, the first division of the sac taking place by a wall which separates 

 it into two nearly equal portions, each of which contains a nucleus and produces at least 

 two daughter-cells. In other cases the upper end of the embryo-sac constitutes the 

 first cell of the endosperm, and appears immediately after the oosphere in it has been 

 fertilised to be divided by a transverse wall into halves, the upper one of which is 

 transformed into endosperm by a series of bipartitions, while no such cell-division 

 takes place in the lower half in Viscuvi, Thesuwi, Lathraea. Rhmanihus, Mazus, 



' The question which was for some time much debated, whether the ' cyathium ' of Euphorbia 

 should be regarded as an inflorescence or as a flower, must be considered to be decided in favour of 

 the former view. 



' Hofmeister in Jahrb. f. wiss. Bot. I, p. 185, and Abhandl. d. K.Sachs. Ges. d. Wiss. VI, p. 536. 

 — Strasburger, Die Angiospermen u. d. Gymnospermen, and Ueber Zellbildung u. Zelltheilung. 



