The chromosomes now pass toward the division centre of the cell or =equatorial plane= and form, collectively, the =equatorial plate= (Plate 1, Fig. 5). At this point of cell division, the chromosomes are =U=-shaped, and the curved part of the chromosomes faces the equatorial plane. The chromosomes finally split into two equal parts (Plate 1, Fig. 6). The actual separation of the halves of chromosomes is brought about by the attached polar fibres, which contract toward the polar caps (Plate 1, Fig. 7). The chromosomes are finally drawn to the polar caps (Plate 1, Fig. 8). The chromosomes now form a rounded mass. They then separate into linin threads and chromatin granules. Nucleoli reappear, and nuclear sap forms. Finally, a nuclear membrane develops. The spindle fibres, which still extend from pole to pole, become thickened at the equatorial plane (Plate 1, Fig. 8), and finally their edges become united to form the =cell-plate= (Plate 1, Fig. 9), which extends across the cell, thus completely separating the mother cell into two daughter cells. After the formation of the cell-plate, the spindle fibres disappear. The cell becomes modified to form the =middle lamella=, on either side of which the daughter protoplast adds a cellulose layer. The ultimate composition of the middle lamella and the composition and structure of the cell wall will differ according to the function which the cell will finally perform. [Illustration: PLATE 1 Nine figures, showing stages in the cell-division common to the onion root (_Allium cepa_, L.)] ORIGIN OF MULTICELLULAR PLANTS All multicellular plants are built up by the repeated cell division of one original cell. If the cells formed are similar in structure and function, they form a tissue. In multicellular plants many different kinds of tissues will be formed as a result of cell division, since there are many different functions to be performed by such an organism. When several of these tissues become associated and their functions are correlated, they form an organ. The association of several organs in one form makes an organism. The oak-tree is an organism. It is made up of organs known as flowers, leaves, stems, roots, etc. Each of these organs is in turn made up of several kinds of tissue. In some cases it is difficult to designate a single function to an aggregation of cells (tissue). In fact, a tissue may perform different functions at different periods of its existence or it may perform two functions at one and the same time; as an example, stone cells, whose primary function is mechanical, in many cases function as storage tissue. The cells forming the tissues of the plant, in fact, show great adaptability in regard to the function which they perform. Nevertheless there is a predominating function which all tissues perform, and the structure of the cells forming such tissues is so uniform that it is possible to classify them.