REPRODUCTION, VEGETABLE. (VEGETABLE OVUM.) Before the mi croscope was placed in the hands of the vegetable physiologist, the conditions by which he was surrounded in the investigation of the processes by which the embryo is formed, differed widely from those which exist at present. From the absence of means of observation, the phenomena of reproduction could only be studied in the Phanerogamia. Even as regards the highest cryptogamous plants, very little had been ascertained ; while the Algm and Fungi were involved in the most complete obscurity. But in the Phanerogamia it was already known that two kinds of organs were essential to the produc tion of the embryo, and something had also been learnt of the mode of their combina tion. No sooner were these facts established, than, with a readiness of which innumerable examples present theniselves in the history of physiological investigations, they were at once seized upon to serve as the ground of a COM-. parison between the animal and vegetable kingdoms ; and naturalists soon passed to the conclusion that the organs in question were of distinct sexes, or, in other words, stood in the same relation to each other as those of animals. The analogy seemed sufficient for the mind to rest upon ; and the doctrine derived from it was received as indisputable.
The influence exercised by the state of thing,s we have just described, may be traced in two directions :—In the first place, a strong tendency is even now observable in the minds of naturalists, especially in this country, to ap proach the subject from the same point of departure as before, 1.vhen the circumstances were so different. The appearance of greater simplicity among the higher plants, was en tirely dependent on conditions belonging ex clusively to the observer ; that is to say, on the imperfection of the means of observation. Now that so many of these imperfections are removed, to take the Phanerogamia as our starting point in approaching either this or any other general question in vegetable physi olou, is evidently unreasonable ; we must commence our investigation where there are fewest complications—namely, at the unicel lular plants. From this point we must ascend
from class to class, following as closely as may be the natural order of complexity of organisa tion.
A second result of the same causes is the confusion which so frequently arises in the employment of terms which are derived from the animal kingdom, such as " male," " female," " ovum," &c. As our knowledge of the sub ject becornes more accurate, the grounds upon which the assumed correspondence between the reproductive organs of plants and animals rests appear less substantial. The only analogies, indeed, which can possess any red value are those occurring between the lowest members of the two series. This is the only point at which the two kingdoms are in mutual contact, and consequently it is here only that an actual correspondence can be traced through succes sive consecutive modifications.
The subject of the following article is the orimin and development of the germ, or, in other words, the reproduction of plants by means of germs.
Considering it as a conclusion, respecting which there can remain very little doubt in the present state of vegetable physiology, that every existing plant must have originated as a single cell, there are two modes in which this may be supposed to have taken place. In the one case, a cell originally forming a part of the tissue of the parent, and not previously dis tinguished in any respect from its neighbours, suddenly assumes a new activity which it did not before possess. To this change the term " Verjlingung," or, as it is rendered by Mr. Henfrey, "rejuvenescence," has been applied, and is most expressive of its nature. A cell in which there has previously been a gradual diminution in the intensity of vital manifesta tions, recovers the capability of development which it possessed when first formed. Now, however, the formative force bv virtue of which the whole subjects the development of all its parts to its own, being abated and weakened by age, the rejuvenescent cell be comes individualised and is transformed into the rudiment of a new plant, in accordance with a capability of development, which resides entirely in itself. This process is called gem ination.