Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/271

PHYTOGENESIS. 237 In the further progress of organization (in which process the gum is always the last, immediately preceding fluid), a quantity of exceedingly minute granules appear in it, most of which, on account of their minuteness, look like mere black points. Iodine then seems to colour the fluid a somewhat darker yellow. The granules, however, when their size is sufficiently large to render their colour perceptible, become of a dark brownish-yellow under its influence.

It is in this mass that organization always takes place, and the youngest structures are composed of another distinct, perfectly transparent substance, which presents an homogeneous colourless mass when subjected to pressure; when dried it imbibes water and swells; it is not at all affected by tincture of iodine, nor does it ever imbibe it; after pressure it appears as colourless as before, and is so completely transparent as to be altogether invisible when not surrounded by coloured or opaque bodies. This substance frequently occurs in plants (for example, in great quantity, together with a little starch, in peculiar large cells in the tubers of Orchis) ; for brevity’s sake I shall call it vegetable gelatine; and am inclined to class under this head, as mere slight modifications, pectine, the basis of gum tragacanth, and many of those substances which are usually enumerated under the term vegetable mucus.

It is this gelatine which is ultimately converted by new chemical changes into the actual cellular membrane, or structures which consist of it in a thickened state, and into the material of vegetable fibre.

I now pass on to our subject itself. There are two situations in the plant in which the formation of new organization may be observed most easily and clearly, in consequence of there being cavities closed by a simple membrane, viz. in the large cell, which subsequently contains the albumen of the seed, the embryonal sac, and in the extremity of the pollen-tube, from which the embryo itself is developed. The embryonal sac never contains starch originally, but probably, in most instances, the saccharine solution (which gives the sweet taste to unripe pod-fruits and the Cerealia), or gum.

The pollen, on the contrary, always contains starch, or the above-mentioned granulous mucus representing it, as an essential constituent part. The so-called vegetable spermatozoa