The pollen-sac thus stands in the same relation to the nucellus as the microsporangium does to the macrosporangium: in the pollen-sacs and microsporangia a _number_ of spores arise by the tetrad-division of several mother-cells; in the nucellus and macrosporangium, a reduction of the cells already formed takes place to such an extent that the number of macrospores becomes one (_Salvinia_, _Marsilia_, Phanerogams) or four (_Selaginella_), or rarely a large number as in _Isoëtes_. In the Ferns, as stated on page 210, etc., _indusia_ covering the sori very often occur. Horsetails and Club-Mosses have no indusium; but in all Phanerogams cupular or sac-like structures (_integuments_) are found which envelop the nucellus. These develope from the upper end of the funicle (_ii_ and _ie_, in Fig. 248; _y_ and _i_, in Fig. 249) and enclose the nucellus on all sides as a sac, leaving only a small channel at the apex of the nucellus--the _micropyle_--(Fig. 249) through which the pollen-tube proceeds to the embryo-sac. The ovules of the Gymnosperms have only one integument (Figs. 251, 264, 269, 274) and the same is the case with the majority of the Sympetalæ and a few Choripetalæ; but the Monocotyledons and most of the Choripetalæ have two integuments (Fig. 249). [Illustration: FIG. 249.--Various forms of ovules: _A_ an erect ovule (_orthotropous_); _B_ reversed (_anatropous_); _C_ curved (_campylotropous_): _k_ the nucellus (shaded in all the figures); _s_ the embryo-sac; _ch_ the base of the ovule (chalaza); _y_ and _i_ the external and internal integuments, the dotted line denotes the place where the scar (_hilum_) will form when the seed is detached from the funicle.] In shape the integuments resemble very closely the cupular indusium of the Hymenophyllaceæ, certain Cyatheaceæ (Fig. 212 _E_), and _Salvinia_ (Fig. 218); that they are really homologous with these is probable, but is not proven. Some authorities regard them as structures found only in the Phanerogams. The ovule is thus a “_monangic_” (_i.e._ reduced to 1 sporangium, the _nucellus_) _sorus_, situated on a funicle, and enclosed by one or two cupular _indusia_, the integuments. Some of the ovules are _erect_ (_orthotropous_), others _curved_ (_campylotropous_), the majority _reversed_ (_anatropous_) (Fig. 249). [Goebel (1884 and earlier) with Strasburger considered the entire ovule of the Phanerogams as homologous with the macrosporangium, the integuments however as new structures in contradistinction to the Ferns: the funicle then corresponds to the stalk of the sporangium. The integuments of the ovule (according to Goebel, 1882) differ from the indusium of the Fern-like plants in being developed from the basal portion of the nucellus and are not, as in the Ferns and _Isoëtes_, a portion (outgrowth) of the leaf which bears the sporangia (_K_).]