Cells of the wood and the bast are easily recognized as plant cells, while muscle and nerve cells are just as characteristically ani mal; but the similarity of the cells of animals and plants, especially in their younger stages, is so great that it easily leads to speculation. In both, the mass of protoplasm and the nu cleus are the essential features. In its ulti mate structure, the protoplasm of animals and plants seems to be essentially identical, although the characteristic vacuoles of plants are incon spicuous or lacking in animals and some of the products of protoplasm are different in the two kingdoms. The nuclei of animals and plants are so identical that in multitudes of cases, even an expert could not say whether a given nucleus belongs to an animal or to a plant. Both have a nuclear membrane, nu clear sap, a nucleolus and chromatin. In both, during division, the chromatin breaks up into a definite number of pieces, called chromo somes, each of which splits longitudinally, and one-half passes to each of the two nuclei formed by the division.
This division of the nucleus and cell to form two new nuclei and cells is one of the most familiar and, at the same time most complicated phenomena of biology. The es sential features of an ordinary division in a vegetative cell are shown in Fig. 3, The chromatin of the resting nucleus (A) forms a more or less continuous thread, called the spirem (B); the spirem segments into a defi nite number of pieces called chromosomes (C), the number being constant for any given spe cies. Each chromosome splits longitudinally into two pieces, forming two new chromosomes (D). The new groups of chromosomes sepa rate (E) and form two new nuclei, while a new cell wall develops between them (F). This whole process of division, formerly called karyokinesis, is now more generally called mitosis. Preceding the formation of eggs and sperms, there are two peculiar divisions, called the reduction divisions, which reduce the number of chromosomes to one-half the number characterizing the vegetative nuclei. This reduction of chromosomes is found in all animals and plants which have reached the level of sexuality. The fusion of the egg and sperm at fertilization restores the number. In both animals and plants, it is becoming recog nized that the chromosomes are the physical basis of heredity.
The lowest animals and plants are unicel lular and, since animals are either immediately or ultimately dependent upon plants, it follows that unicellular plants were the first forms of life. The single cell performs all the func
tions, ingestion, digestion, assimilation, excre tion,. respiration, reproduction, etc. The di versity of form attained by these unicellular organisms is amazing.
The first step in the evolution of the plant body was attained when, after the division of a cell, the two resulting cells failed to separate. Continued division of this sort results in the formation of a filament, either simple or branched, but still consisting of a single row of cells. The next step is attained when divi sions take place in two directions, so that, in stead of a filament, a flat plate, one cell in thickness, is built up. In the filaments and in such thin plates there is little or no differen tiation among the vegetative cells, except that some may be more or less modified as hold fasts to attach the plant to a substratum; but in both there may be considerable differentia tion of reproductive cells. When divisions occur in three directions, more or less massive bodies are built up and the division of labor is accompanied by extreme differentiation among the various cells. Even in rather small bodies, the outer layer of cells is likely to be modified as a protective layer. If the body is so large that the inner cells are at a considerable dis tance from the surface, some cells will be modi fied for conduction. In large bodies, some cells will be modified to give support and, finally, in complex organisms, the various functions will be performed by the cells which have become modified in various ways. It is interesting to note that these cells which be come so variously modified in their later de velopment are, in their early stages, very much alike; e.g., in the lily the cells which are to form the epidermis, the digesting, assimilat ing, conducting and even reproductive tissues, are strikingly similar. What causes cells which appear to be exactly alike, when first formed, to develop along different lines, so that some become elongated while others remain short, some become thick walled while others remain thin walled, some function in digestion, others in conduction, etc., is still an unsolved prob lem. The egg cells of the oak, the lily and the sunflower are almost indistinguishably similar and the early stages in the development of their embryos are not very different. What de termines the development is a great problem still unsolved. To say that heredity is the cause merely gives a name to the difficulty.