AN ENCYCLOPEDIA OF HORTICULTURE. 



539 



Ovule continued. 



varies much; in some Ovules it is very long (see Fig. 

 805), while others do not possess it. Most Ovules have 

 it closely adherent to one side, the Ovule being in- 



ANATROPOUS OVULE OF ACT.EA SPICATA IN SECTION. 

 Only one Coat exists on it. 



verted, or anatropous, as in Actcea, so as to bring the 

 micropyle close to the placenta, while the Ovule itself 

 remains straight (see Fig. 804); other Ovules are curved 



FIG. 805. YOUNG CAMPYLOTROPOUS OVULE OF LYCHNIS DIURNA 

 IN OPTICAL SECTION. The Funiculus is very long, and the 

 Inner Coat projects considerably at the Micropyle. 



or campylotropous (see Figs. 805 and 806), as in the 

 Pea and in Lychnis ; so that in them also the micropyle 

 is brought near the placenta, but the funiculus is only 



FIG. 806. CAMPYLOTROPOUS OVULE IN SECTION, showing 

 Two Coats. 



adherent slightly to one side of the Ovule. The form 

 selected for description is called erect, or orthotropous 

 (see Fig. 803). The Ovules of orchids have no fibro- 

 vascular bundle in the funiculus. 



Development. This shows peculiarities in certain plants. 

 In the Ovules of Santalum album, and occasionally in 

 those of some orchids, there are two embryonal vesicles, 

 BO that two embryos may form in a seed. In seeds of 



Ovule continued. 



Oranges, of the Spindle-tree, of Funkias, and of certain 

 other plants, it is usual to find two, or even three or 

 four, embryos. But this results from budding from the 

 tissues of the nucellns of several small cellular growths, 

 which push the wall of the embryo-sac before them, and 

 occupy the place of the true embryo : the latter is 

 usually crushed by them, and perishes. This process has 

 been called Polyembryony. It is a case of vegetative 

 replacing true sexual reproduction, and, by its discovery 

 two or three years ago, various apparent anomalies in 

 hybridisation have been explained. 



The processes of fertilisation, and of subsequent de- 

 velopment, in the Conifers and other Gymnosperms, are 

 BO different in many respects from those above described, 

 as to call for a brief notice. The Ovules are not in- 

 closed in carpels, as in other flowering plants ; but in 

 general appearance and structure they do not show any 

 marked peculiarities as contrasted with the Ovules of 

 these plants. They are orthotropous in some (Yew), and 

 anatropous in others (Pines, &c.). They have only one 

 coat. The contents of the embryo-sac, and the develop- 

 ment of the embryo, are markedly different from what 

 we find in other Phanerogams. At an early stage, the 

 sac is filled with protoplasm, in which lies a single 

 nucleus ; but this very soon divides repeatedly (as in 

 other Phanerogams to form the endosperm), and the sac 

 may thus become more or less full of new cells, the 

 temporary endosperm, ranged along the wall, or occupy- 

 ing most of the cavity. Certain of these cells near the 

 micropyle do not divide like the others, but remain 

 larger than them. Each of these large cells after a 

 time divides into a small cell above and a larger one 

 below. The small cell may remain undivided, but usually 

 divides into a rosette of four (e.g., Juniper), or into more, 

 arranged in two or three layers (e.g., Pines), forming the 

 neck, or rosette cells. In each case, an opening is left 

 between them down to the larger central cell. The whole 

 body formed by the group of cells is known as the 

 corpuscnlum. The number of corpuscula varies from 

 three to fifteen. The pollen grain lies directly upon the 

 micropyle, with no apparent effect, in most cases, for a 

 long time; but at last it pushes a tube down to the 

 embryo-sac, which is pierced, allowing the end of the 

 tube to lie directly on the rosette cells, and to push 

 between them into the upper end of the large central 

 cell. Some of the substance passes into the latter cell, 

 part becoming diffused, and part remaining visible for a 

 time as an upper nucleus ; but this nucleus afterwards 

 becomes united with the true nucleus of the cell, to 

 form a new nucleus. Then follows a series of cell- 

 divisions at the lower end of the central cell, which 

 result, in general, in the formation of four rows of cells, 

 side by side, lengthwise. The lowest cell of each row 

 divides, to form an embryo. Thus, it is the rule for 

 Gymnosperms to produce several embryos in a seed ; but 

 in most seeds only one embryo is fully developed, the 

 others being destroyed by pressure of the one upon 

 them. This one developed is nourished largely at the 

 expense of the endosperm. 



Much use is made in Systematic Botany of the differ- 

 ences in structure, form, and development of and in 

 Ovules. 



The meaning of the lettering in the sections of Ovules 

 is as follows : ft, outer coat ; b, inner coat ; c, nucellus ; 

 d, embryo sac ; e, micropyle ; /, stalk, or funiculus ; g, fibro- 

 vascular bundle of stalks ; ft, chalaza ; t, placenta. 



OXALIDEJE. A tribe of Geraniacece. 



OXALIS (from oxys, acid ; referring to the taste of 

 the leaves). TRIBE Oxalidece of OED. Geraniacece. 

 A very large genus (upwards of 220 species) of green- 

 house, hardy, or nearly hardy herbs, sometimes stem- 

 less, with a bulbous or fleshy rhizome, s 



