OVULE. 



[ 482 ] 



OVULE. 



the growth of the embryo, this ultimately 

 filling the embryo-sac. In the Nymphse- 

 acese, however, these cells remain, forming 

 an inner ENDOSPERM or ALBUMEN, in 

 addition to that formed from the body of the 

 nucleus. In other cases (those of exalbumi- 

 nous seeds) the embryo not only displaces 

 these internal endosperm-cells, but in the 

 course of its growth causes the absorption 

 of the tissue of the nucleus, and ultimately 

 constitutes the entire seed, enclosed only by 

 the true integuments. The remaining cha- 

 racters are given under ALBUMEN and EM- 

 BRYO. We must not omit to notice the 

 views entertained by Schleiden with regard 

 to the origin of the embryo. This author 

 believes that the embryonal vesicle is the 

 swollen and subsequently detached end of 

 the pollen-tube, which enters the embryo-sac, 

 and progressing to a variable distance, there 

 itself constitutes the suspensor. Only one 

 author, Schacht, perseveres with Schleiden 

 in maintaining this view. Tulasne, however, 

 is in doubt whether the germinal vesicles 

 exist before the pollen-tube enters the micro- 

 pyle. We have certainly seen them before, 

 but entertain great doubt whether they 

 possess a cellulose coat before impregnation. 

 Recent observations on the ovule ofSantalum 

 album lead us to imagine that they receive 

 the influence of the pollen while in the state 

 of nucleated protoplasmic corpuscles, analo- 

 gous to the unimpregnated spores of Fucus. 

 In the Gymnospermous Flowering Plants 

 (Coniferse, "&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 directly. 

 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 (endosperm-) 

 cells are formed. In many of the Abietineae 

 this goes on until the spring following the 

 impregnation. Ultimately the embryo-sac 

 is found to have increased to more than 

 twenty times its original size, with the endo- 

 sperm-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 en- 



larged ones (secondary embryo-sacs) and 

 the wall of the original embryo-sac, become 

 divided by two perpendicular septa standing 

 at right angles into four cells. A central 

 intercellular passage then appears at the 

 contiguous angles of these four cells. These 

 new bodies, which closely resemble the 

 archegonia of the LYCOPODIACE^E, were 

 called corpuscula by Mr. Brown, who dis- 

 covered them. 



Free cells are next formed in the secondary 

 embryo-sacs of the corpuscula, several at 

 the upper, one 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 be- 

 comes divided into four collateral cells, these 

 multiply again, and subsequently the cellular 

 body (proembryo) so formed, breaks through 

 the base of the secondary embryo-sac, and 

 grows down in the substance of the lower 

 part of the nucleus, which is now in a state 

 of semi-solution. The proembryo then se- 

 parates into four cords (corresponding to its 

 four primary cells), and these filaments (sus- 

 pensors) terminate in rounded cells, which 

 divide into four, each of which is an em- 

 bryonal vesicle ; 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 per- 

 fect embryo is found lying in a mass of albu- 

 men formed of the nucleus, but its radicle, 

 developed at the point of junction of the 

 suspensor, never becomes very clearly defined 

 at its extremity, but remains organically 

 continuous with the albumen. It should be 

 stated that Schleiden. and Schacht affirm 

 here also that the proembryos are really 

 formed from the extremities of the pollen- 

 tubes which enter into the archegonia. 



Other points relating to the development 

 of ovules will be found under POLYEM- 

 BRYONY, SEEDS, and CELL-FORMATION. 



The methods of investigating the develop- 

 ment of ovules are simple in their nature 

 but rather difficult in practice. The ordi- 

 nary plan is to place an ovule between the 

 thumb and fore-finger of the left hand, and 

 with a very sharp razor cut it into two un- 

 equal 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 



