OVULE. 



[ 668 ] 



OVULE. 



vesicles exist before the pollen-tube enters 

 tlie micropyle. We have certainly seen 

 them before ; but we believe they do not 

 possess a cellular coat before impregnation. 

 Observations on the ovule of Santalum 

 album have led us to conclude that they 

 receive the influence of the pollen -^hile in 

 the state of nucleated protoplasmic cor- 

 puscles, analogous to the unimpregnated 

 spores of Fucus; and this view has since 

 been supported by the later observations of 

 Schacht, although Hofmeister and Radlko- 

 fer maintain that the germinal bodies possess 

 a cell-membrane before impregnation. 



Li the Gymnospermous Flowering Plants 

 (Couiferse, &c.) the ovule, consisting of a 

 cellular nucleus and a single coat, is placed 

 upon an open carpel, and its widely-open 

 micropyle receives the pollen-grain. At 

 the period of impregnation, the embryo-sac 

 is a cavity deeply seated in the tissue of 

 the nucleus ; it is formed by the coalescence 

 and expansion of several cells (in the Yew 

 there are often at first three embryo-sacs). 

 In the embryo-sac a number of free nuclei 

 soon appear, and numerous free (endo- 

 sperm-) cells are formed. In many of the 

 Abietinese this goes on imtil the spring 

 following the impregnation. Ultimately 

 the embryo-sac is found to have increased 

 to more than twenty times its original size, 

 with the endosperm-cells applied in layers 

 over the inside of its walls, increasing in 

 number until the cavity is filled up. Then 

 a certain number of cells (from three to 

 eight in different genera), situated near the 

 micropyle end, but each in the layer next 

 but one to the wall of the embryo- sac, 

 become enlarged, and the cells intervening 

 between these enlarged ones (secondary 

 embryo-sacs) and the wall of the original 

 embryo-sac become divided, by two per- 

 pendicular septa standing at right angles, 

 into four cells. A central intercellular pas- 

 sage then appears at the contiguous angles 

 of these four cells. These new bodies, which 

 closely resemble the archegonia of the Lv- 

 copoioiACE^, were called corpuscula by 

 Brown, who discovered them. 



Free cells, or perhaps merely protoplasmic 

 masses, are next formed in the secondary 

 embryo-sacs of the corpuscula, several at 

 the upper, oiie at the lower end. The pol- 

 len-tubes now advance, breaking down the 

 tissue of the nucleus, until their points 

 reach the corpuscula ; and one then makes 

 its way down the intercellular canal of 

 each, to reach its secondary embryo-sac ; 



the free cell at the base of this (germinal 

 vesicle) then becomes divided into four col- 

 lateral cells ; these multiply again ; and sub- 

 sequently the cellular body (jiroembri/o) so 

 formed breaks through the base of the 

 secondary embryo-sac, and grows down in 

 the substance of the lower part of the nu- 

 cleus, which is now in a state of semisolu- 

 tion. The proembryo then separates into 

 four cords, corresponding to its four primary 

 cells ; and these filaments (suspensors) ter- 

 minate iu rounded cells, each of which is 

 an embryonal cell; so that there are now 

 four times as many rudimentary embryos 

 as there are corpuscula. Out of all these, 

 only one ultimately remains and becomes 

 perfectly developed ; the rest are absorbed 

 during the ripening of the seed. In the 

 latter, the perfect embryo is found Ipng in 

 a mass of alljumen formed of the nucleus ; 

 its radicle, developed at the point of junc- 

 tion of the suspensor, never becomes very 

 clearly defined at its extremity, but remains 

 organically continuous with the albumen. 



Other points relating to the development 

 of ovules will be found under Polyeji- 

 BRYONY, Seeds, and CELL-formation. 



The methods of investigating the deve- 

 lopment of ovules are simple in their nature, 

 but rather difficult in practice. The ordinary 

 plan is to place an ovule between the thumb 

 and fore finger of the left hand, and with a 

 \tiry sharp lancet cut it into two unequal 

 pieces, in the direction of the axis. The 

 larger of the two being then laid on its flat 

 side on the finger (by the aid of a mounted 

 needle), another slice is made so as to leave 

 a section preserving all the central part of 

 the ovule. This adheres either to the finger 

 or the lancet ; and a di'op of water should be 

 placed on it to free it ; then it may be trans- 

 ferred to a slide with a very fine cainel's- 

 hair pencil. Examined under a low power 

 (a half-inch), it will probably be found to 

 require further dissection, with exceedingly 

 fine needles, under a simple lens: some- 

 times mere pressure is of service. For the 

 minute details, the quarter and eighth ob- 

 ject-glasses will require to be applied. We 

 have found ovules which have been kept in 

 spirit easier to dissect ; when fresh, the cell- 

 membranes are excessively delicate. It need 

 scarcely be added that ovules require to be 

 examined in all stages in order to imder- 

 stand their developmental characters; and 

 the student must not be disheartened by the 

 failure of a large proportion of his sections 

 to ali'ord satisftictorv observations. 



