In the other case, the cell from which the new plant originates, manifests from the first moment of its existence conforrnity to law, on the one hand, in its anatomical relations to the organs of the parent upon which it is supported, or within which it is enclosed, on the other, in the mode in which its develop ment commences — its transformation being the result of an activity inherent in it, not as an individual cell, but as being a part of the parent, and still under the control of its formative force. It is to this cell that the name gerra is alone applicable in the restricted sense in which it is generally used ; namely, as expressing not only that it will, if it live long enough, transform itself into an embryo, but that it presents itself uniformly in the same species under the same anatomical condi tions.
The term " vegetable ovum," placed at the head of this article, is employed in order to connect it with the preceding one on the " animal ovum," to which it is intended to form a sequel. In its usual acceptation in vegetable physiology, the word means the generative product of the Phanerogamia only. And even if we were to extend its meaning so far as to include all those varieties of germ, for the development of which two organs mutually dependent on each other for the ac complishment of their reproductive functions are necessary, we should still be obliged to disregard one half of the vegetable kingdom.
pear-shaped, fusiform, or oval ; at the same time they are endowed with the power of active motion, and are furnished with a pair of vibratile cilia, emanating from their anterior ALGtE, FUNGI, AND LICHENS.
1. Reproduction by mecins of Zoospores.— Among the most simply organised infusory animals are included several genera, which are admitted hy all naturalists to present, in the aggregate of their characters, as many points of resemblance with plants as with animals. They agree with plants in their chemical con stitution, in the mode in which they react on the atmosphere, and in their green colour. The Euglena viridis, which is so common in all our shady ponds, though in active motion during the greater part of its life, manifests at other periods a condition of plant-like repose. The contractility displayed in its rapid and ever-varying changes of form is a property which, there can be little doubt, manifests itself frequently among undoubted plants ;* so that the transition from the Eu glenas to many of the forms of the Proto coccus-like Algm is almost insensible. The elaborate researches of Cohn on the so-called Protococcus pluvialis, have unfolded many facts of the greatest importance in relation to this subject. The well-known permanent form
of this plant is that of a globular cell, furnished with a distinct colourless membrane, and con taining in its interior a semifluid protoplasma, in which numerous green or red granules are embedded. Cohn found that when water is added to Protococci in this condition, they immediately become the subjects of an active reproductive process. In the interior of each cell are formed, by the division of its contents, secondary cell-like bodies, the number of which is always either two, or a power of two. These bodies, which possess no distinct mem brane, either give rise to stationary cells similar to their parent, or, as is by far more frequently the case, especially when the number of newly produced individuals is large, they become extremities. In the course of their further development, these actively moving bodies, which we shall call Zoospores, become invested with a distinct membrane. This seems to be a preparatory step to the cessation of their movements; for shortly afterwards they are observed to lose their vibratile cilia, and as sume a form which. corresponds more or less completely to that of the mother cell. In many cases, however, before this result is ac complished, a second reproductive process commences in the still ciliated zoospore. A division of its protoplasmic contents, similar to the first, takes place, and a second genera tion of zoospores is set free, each of which is capable, after exhibiting active motion for a longer or shorter period, of becoming a sphe rical, motionless cell, in all respects similar to the original parent. Thus an individual Pro tococcus in its stationary form, may reproduce itself either directly, or with the intervention of a second generation. In the former case, the germ may either become at once an indi vidual similar to its parent, or may pass through a preparatory period, during which it is not only provided with motor organs, but manifests in the protoplasm of which it is formed, a property of contractility resembling that of animals. Facts similar to the above are descrbed by Braun as occurring in another unicellular Alga (Ascidium acuminatum). This species, which is found attached to stones or other objects, resembles the Protococcus plu vialis in its general form. By the division of the protoplasma which lines its cell-wall, numerous zoospores take their origin. These are pear-shaped, and at the apex of each is observed a pair of vibratile cilia.