382 



FOURTH GROUP. — SEED-PLANTS. 



nucellus ; most gamopetalous Dicotyledons and some other plants have only one 

 integument ; nearly all INIonocotyledons have two. and sometimes a third integument is 

 subsequently formed, the aril, as in Myristica, Euonymus, Asphodelus luteus and Aloe 

 stibtuberculata. The ovule is often straight or orthotropous when it is the terminal 



structure of the floral axis, and the funicle 

 remains short, as in the Piperaceae and Polygo- 

 naceae ; it is comparatively rarely campylotropous, 

 in which case the nucellus with its integuments is 

 itself curved, as in the Gramineae, Caryophyllaceae 

 and others ; its usual form in Angiosperms is the 

 miatropous, in which the nucellus with its integu- 

 ments is inverted, being turned round on the 

 summit of the funicle towards its base, to which 

 the micropyle is therefore directed (Figs. 299 ^, 

 300) ; in this case the funicle where it runs along 

 one side of the ovule and adheres to it is termed 

 the raphe. The micropyle, especially in the 

 Monocotyledons, is often formed by the inner 

 integument {secundiiic) only projecting above the 

 nucellus ; sometimes, especially in the Dicoty- 

 ledons, the outer integument {primine) grows up 

 above the mouth of the inner one, and the 

 micropylar canal is then formed at its outer 

 extremity, the cxostome, by the outer integument, 

 at its inner portion, the endosiof?ie, by the inner. 

 If there are two or three integuments, the innermost 

 is always formed first, and then the outer, and lastly 

 and usually much later the third, the aril; the 

 integuments therefore in respect to the axis of the 

 ovule are formed in basipetal succession. The 

 transverse zone from which the one or two true 

 integuments spring is termed the chalaza, or base 

 of the ovule. The integuments are usually only 

 a few layers of cells in thickness, and appear 

 like membranes, especially when they enclose a 

 large nucellus (Fig. 299 E, /) ; but if only one integument is formed, the nucellus 

 usually remains very small, while the integument is thick and solid and far overtops 

 the nucellus, forming the chief mass of the ovule before fertilisation, as in Hippuris 

 (Fig. 272), the Umbelliferae (Fig. 300) and Compositae (Fig. 310); see also Fig. 317. 

 The history of the development of the ovule is as follows ^ The ovule makes 

 its first appearance as a small protuberance, which either occupies the apex of the 

 floral axis or originates in a group of cells of the placenta ; the epidermis together 



Fig. 314. Bvassica Naptis. Flower after re- 

 val of the sepals and petals ; a anthers.y filaments, 

 in the form of club-shaped 



1 Wanning, De I'ovule .Ann. d. sc. nat. 1878). — Strasburger, Die Angiospermen u. d. Gymno- 

 spermen, 1879 — Jonsson, Om embryosackens utveckling hos Angiospermernae (Lunds Univ. Ars- 

 skrift, T. XVI). 



