A handbook of systematic botany

by Eugenius Warming

Fig. 248.—Development of the ovule in the Red Currant, Ribes rubrum, arranged alphabetically in the order of development. A Is the youngest stage, E the oldest. ii Inner integument; ie outer integument; nc nucellus; m archespore (mother-cell of the embryo-sac).

The Ovule in the Phanerogams arises most frequently on a projecting portion of the carpel, termed the placenta. The ovules (compare the sporangium of the Eusporangiatæ and especially the pollen-sac) take their origin from a group of cells which lies beneath the epidermis (Fig. [248] A, B). First of all a small papilla is formed, which is later on provided with a vascular bundle and becomes the funicle; this probably has the same value as the projections (“placenta”) on which the sori in the Ferns are attached. Only one macrosporangium (nucellus; Fig. [248] nc) is developed at the apex of the funicle. This arises by a process of cell-division exactly corresponding to that by which the pollen-sacs are formed (Fig. [248] C-E), with this difference only, that while a great many cells may be distinguished in each pollen-sac, which forms pollen-grains by tetrad-division, only a few are found in the ovule, and all these moreover are suppressed, with one single exception which developes into the macrospore (embryo-sac) without undergoing a division into tetrads. The wall of the embryo-sac, in the Gymnosperms, may be thick and divided into two layers and partly cuticularized, as in the spores of the Cryptogams which are to be set free. In the Angiosperms, on the other hand, the wall is extremely thin.

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]).

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).]