ANGIOSPERMS. 



57^ 



When there are two or three integuments, the innermost (the Primine of Mirbel) 

 is generally formed first, then the outer one (the Secundine), and finally, usually at 

 a much later period, the Aril ; the order of development is therefore basipetal in 

 reference to the axis of the ovule. The transverse zone from which the single 

 or the two true integuments spring is termed the Chalaza (more correctly the base 

 of the ovule). 



The integuments are usually only a few layers of cells in thickness, and have 

 the appearance, especially when they enclose a large nucellus, of thin membranes 



Fig. 397.— /—K// stages of development of the ovule of Orchis militaris (X 55") ; I— VI seen from the side in longitudinal 

 section, VII from the front, the funiculus being behind, VIII a horizontal section of /; x x the axial row of cells, the upper one 

 of which is the archesporium, ^.ythe funiculus, ii the inner, ai the outer integument, K the nucellus^ es the micropyle, h an 

 intercellular space : in VII the embryo-sac e has completely replaced the tissue of the nucellus. 



(Fig. 382, E). But when only one integument is developed, the nucellus usually 

 remains very small, while the integument becomes thick and solid, extending far 

 beyond the nucellus, and forming, before fertilisation, the principal mass of the ovule, 

 as in Hippuris (Fig. 360), Umbelliferae (Fig. 383), and Compositae (Fig. 393). 



There is still much doubt about the history of development of the separate 

 parts of the ovule ^ ; the following may be stated as certain or at least probable. 

 In the formation of the erect orthotropous ovule the apex of the floral axis rises 

 within the ovary as a roundish or conical ovoid protuberance which forms the 



[For an account of the development of the ovule see infra, p. 576. 



