compounds which can serve the organism as food. Thus plant cells which contain chlorophyll bodies differ from all other cells in manufacturing their own food and in not being obliged to obtain it from without. Since in all or nearly all plants the lack of chlorophyll, when it is lacking, is due to degeneration in consequence of the acquirement of a saprophytic or parasitic mode of life, the possession of chlorophyll bodies and the consequent food-forming power constitute the most real distinction which separates plants from animals. Treated understandingly, this affords the most satisfying response to the ever-recurring demand for a statement of the differences between the two organic kingdoms, although the distinction is no more an absolute one, as shown by the case of the fungi, than any of the other less important ones often suggested.
We have seen that most plant cells possess firm walls, and it is little more than a decade since plants were generally believed to consist of blocks of protoplasm quite shut off from each other, in most cases, by the surrounding walls. The many difficulties entailed by such belief, and the impossibility of explaining the transfer of substance or the transmission of stimuli in certain tissues, was the chief incentive to Gardiner's researches. This author and others after him have shown that, in most tissues, and especially just where they are needed to explain observed phenomena, tiny threads of protoplasm penetrate the cell walls, connecting the protoplasmic masses of neighboring cells and forming the means of communication between them. So that we no longer think of the cells of a multicellular plant as isolated masses of protoplasm, but as connected masses, while the intervening walls give the necessary rigidity and resistance to the tissue.
Passing now to the nucleus of the cell, we find a complicated structure. Surrounded by undifferentiated protoplasm, it is bounded against it by a very delicate "nuclear membrane." Within this is a loose network of somewhat solid substance, whose meshes are believed to be filled by a clear, structureless fluid. In this lie one or more small globular masses of a very strongly refractive substance, known as nucleoli. That the nucleus is the controlling organ in the more active cell processes is indicated by many facts. It has been found that a cell from which the nucleus has been removed is unable to grow or to form new cell wall. In cells in which growth or any active process is taking place at some definite point, the nucleus takes a position near to that point, although thus lying far from the center of the cell. Any shifting of the point of greatest activity is accompanied by a corresponding change in the position of the nucleus. The centrospheres lie ordinarily close beside the nucleus and play their chief rôle in connection with its division, which we may proceed to discuss.