STRUCTURE OF OVULES FERTILIZATION. 399 



mount them either dry, or (if they will bear it without rupturing) 

 in fluid. The most delicate and interesting forms are found, for 

 the most part, in plants of the Natural families Amarantacece, 

 Ciehoracece, Cucurbitacece, Malvaceae, and Passiflorece ; others are 

 furnished also by Convolvolus, Campanula, (Enothera, Pelargonium 

 (Geranium), Polygonum, Sedum, and many other Plants. It is 

 frequently preferable to lay down the entire anther with its ad- 

 herent pollen-grains (where these are of a kind that hold to it), 

 as an opaque object; this may be done with great advantage in 

 the case of the common Mallow (Malva sylvestris) or of the Holly- 

 hock (Althcea rosea) ; the anthers being picked soon after they 

 have opened, whilst a large proportion of their pollen is yet un- 

 discharged, and before they have begun to wither, being laid 

 down as flat as possible between two pieces of smooth blotting- 

 paper, then subjected to moderate pressure, and finally mounted 

 upon a black surface. They are then, when properly illuminated, 

 most beautiful objects for the 2 in. objective. 



255. The structure and development of the Ovules that are 

 produced witjiin the ovarium at the base of the pistil, and the 

 operation in which their fertilization essentially consists, are 

 subjects of investigation which have a peculiar interest for scien- 

 tific Botanists, but which, in consequence of the special difficul- 

 ties that attend the inquiry, are not commonly regarded as within 

 the province of amateur Microscopists. The ovule, in its earliest 

 condition, is, like the anther, a mass of cells, in which no part is 

 differentiated from the rest ; gradually, this body, which is 

 termed the nucleus, is found to be enveloped in one, two, or 

 three coats, which are formed by the multiplication of cells that 

 at first constitute merely an annular enlargement at its base ; 

 these coats, however, do not entirely close in around the nucleus, 

 at the point of which a small aperture always remains, that is 

 called the micropyle. In the interior of the nucleus a large cavity 

 is formed, apparently by the enlargement of one of its cells at the 

 expense of those which surround it; and this cavity, which is 

 called the embryo sac, is at first filled only with a liquid proto- 

 plasm. Some little time before fecundation, however, there are 

 seen in it a certain number of free cell-nuclei, rarely fewer than 

 three, and frequently more ; around these, free cells of a sphe- 

 roidal form are developed, which are " germ-cells," of which one 

 only, the embryonal vesicle, is ordinarily destined to be fertilized. 

 This act is accomplished by the penetration of the pollen-tube, 

 which, when it has made its way down to the ovarium, enters 

 the "micropyle" of the ovule, and impinges upon the apex of 

 the " embryo sac," which it sometimes pushes before it, in such 

 a manner as to have given origin to the idea that the tube enters 

 its cavity : no such penetration, however, really exists. By Prof. 

 Schleiden, and his disciple Schacht, it is affirmed that the embryo 

 makes its first appearance within the dilated extremity of the 

 pollen-tube, which speedily becomes filled with a mass of cells ; 



