538 



THE DICTIONARY OF GARDENING, 



Ovule continued. 



that remains straight from its earliest appearance till 

 its full maturity, such as is found in the Docks and 

 the Buckwheat. The Ovule first appears like a small, 



FIG. 801. THREE STAGES IN GROWTH OF ORTHOTROPOUS OVULE. 

 A, Nucellus alone ; B, Inner Coat ; C, Both Coats growing 

 up to cover Nucellus. 



rounded or egg-shaped swelling (see A, Fig. 801), on the 

 placenta, made up of cells alike in form and contents. 

 This body is usually termed the " nucleus " of the Ovule ; 

 but the term " nucellus," now coming into use, is prefer- 

 able, since " nucleus " should be restricted to the body in 

 the protoplasm of living cells generally known under 

 this name. Bound the base of the nncellns a cellular 

 ring (see B, Fig. 801), soon begins to appear, and con- 

 tinues to grow up to form a thin covering around that 

 body. But, while this has been growing, another cover- 

 ing (see C, Fig. 801), forms over it also from below 

 upwards, and the two grow till they leave only a 

 minute opening through them at the upper end (i.e., 

 farthest from the placenta), called the "micropyle," 

 which allows the entrance of a slender tube from the 

 pollen grain. The two coverings are named the inner 

 and the outer coats. At the end next the placenta (see 



Ovule continued. 



has become superseded in the higher plants. In the 

 protoplasm between them lie one or two nuclei, which 

 have to assist in the formation of the endosperm while 



t... 



li 

 FIG. 802. ORTHOTROPOTTS OVULE IN SECTION, showing Two Coats, i 



Fig. 802), there is, generally, a narrower neck, or there [ 

 may be a well-formed stalk, or funiculus ; though some- 

 times there is no visible stalk, the Ovule being sessile, 

 or fixed by a broad base. From the woody, or fibro- 

 vascular, bundle in the placenta, a branch runs into the 

 stalk of the Ovule. The part of the Ovule where the 

 coats are grown to the nucellus is the chalaza. At this 

 part, separation takes place when the seed is ripe, and 

 falls from, the stalk ; and a scar is left, known as the 

 hilum. In the nncellus, a change goes on which results 

 in the formation of a cell the embryo-sac very much 

 larger than the others. It lies near the micropyle, from 

 which, in most Ovules, it is separated by one or two 

 layers of cells of the nucellus. It may be almost as long 

 8 the nncellns, and may, in course of growth, crush the 

 latter, until all its tissue disappears. 



This cell is full of protoplasm ; and in the latter may 

 be seen several bodies, as follows (see Fig. 803) : At the 

 end next the micropyle are three cells, viz., two rather 

 long ones, known as helper cells ; and, at their lower end, 

 a round cell the embryonal vesicle from which the 

 embryo is afterwards developed. At the opposite end 

 of the embryo-sac lie three small bodies the antipodal 

 cells the use of which is unknown. They are believed 

 to represent a structure that plays an important part 

 in the development of Ferns and their allies, but that 



FIG. 803. EMBRYO SAC OF ANGIOSPERM BEFORE FERTILISATION, 

 IN THREE STAGES, showing (h) Helper Cells, (e. v.) Embryonal 

 Vesicle, (a) Antipodal Cells, and (H, n) Nuclei. In A, the 

 Helper and Antipodal Cells, and the Embryonal Vesicle, are 

 still angular, and the two Nuclei are wide apart; in B, the 

 Embryonal Vesicle is rounded, and the Nuclei lie close to- 

 gether ; in C, the Embryonal Vesicle is ready to be fertilised ; 

 the two Nuclei are united to form one, and the Antipodal, as 

 well as the Helper Cells, are rounded off. 



the embryo, or young plant, is being formed. A brief 

 account of the development of the Ovule into the seed 

 will help to make these various structures more in- 

 telligible. 



When the stigma is ready to receive the pollen grains, 

 and one of these is placed upon it, the grain very soon, 

 in most plants, pushes a slender pollen tube between 

 the cells of the stigma, down the loose " conducting 

 tissue" of the style, and into the cavity of the ovary, 

 where it finds its way to the micropyle of an Ovule. 

 Passing down this, it pushes the thin coat of the embryo- 

 sac before it, presses between the helper cells, and reaches 

 the embryonal vesicle. It is believed that a part of 

 the protoplasm passes from the tube into one of the 

 helper cells, which then acts in turn upon the vesicle. 

 The helper cells, and, probably, the antipodal cells, dis- 

 appear ; and the embryonal vesicle begins to grow, and 

 to be divided by cell-walls, so that a rounded mass of 

 tissue is formed, attached above to a row of cells, or 

 to a mass in some plants, called the saspensor. The 

 lower rounded mass is the embryo, which increases in 

 size, and shows beginnings of the axis and leaves, or 

 cotyledons. These parts can easily be made out in most 

 plants. In the meanwhile, after fertilisation has been 

 effected, the nuclei in the protoplasm begin to divide, 

 and to form new nuclei, around which cells form, and 

 the embryo-sac becomes filled, more or less completely, 

 with cellular endosperm. This may disappear in the 

 ripening of the seed (e.g., in Beans) ; or it may remain, 

 and may form a large part of the seed (e.g., in Wheat), 

 in which case it is often called the albumen of the 

 seed. 



DEVIATIONS FROM THE ABOVE TYPE OF STRUCTURE 

 AND DEVELOPMENT. 



Structure. In many plants (e.g., Actcea, Composites, 

 &c.) there is only one coat on the Ovule (see Fig. 804) ; 

 in a few (e.g., Mistletoe) there is no coat, the nucellus 

 being exposed in the cavity of the ovary. The fnniculus 



