442 Morphogenetic Factors 



mitotic figure is formed. Polysomaty of this type has been reported in 

 other cases, as in Cucumis (Ervin, 1941). 



In many instances it has been shown that a process of endomitosis 

 takes place in certain cells by which the chromosome number is doubled 

 (or further multiplied) even though the cell is mature, the nuclear 

 membrane intact, and no mitotic figure has been formed (Geitler, 1949). 

 That this doubling has taken place is indicated by the fact that the num- 

 ber of visible chromocenters, presumably corresponding to the chromo- 

 somes, is doubled. The chromosome number may be definitely determined 

 by inducing these cells to go into typical mitosis, either through wound- 

 ing or by application of growth substances. Under such conditions the 

 chromosomes can readily be counted. Grafl (1939) was thus able to 

 prove that among mature and normally differentiated cells of Sauromatum 

 guttatum some were tetraploid, some octaploid, and some 16-ploid. This 

 situation has now been found in many other cases. D'Amato (1950), by 

 the use of 2,4-D, observed it in roots of a number of monocotyledons, 

 and Holzer (1952), by treatment with indoleacetic acid, in the roots of 

 27 species of angiosperms. Holzer found that the distribution of these 

 polyploid cells was not at random but formed a pattern which was similar 

 in groups of related plants. Often it is not the number of chromosomes 

 (or chromosome centers) that increases but the number of strands per 

 chromosome. 



The connection between polyploidy and the volume of cell and 

 nucleus has important morphogenetic implications. In certain animal 

 tissues, among them the cells of the developing salivary glands of 

 Drosophila, it has been observed that nuclear volume falls into definite 

 classes, each approximately twice the volume of the one next below it. 

 Such "rhythmic" distribution was observed in plants by Monschau ( 1930). 

 The relation between polysomaty and nuclear volume has now become 

 well established (Bradley, 1954, and others). This makes it possible to 

 determine with some accuracy the degree of polyploidy in a mature and 

 differentiated cell by measuring the volume of its nucleus or even of the 

 cell itself, in comparison with related cells and ultimately with those in 

 which the chromosome number can be determined directly. Thus in 

 various angiosperm species, Tschermak-Woess and Hasitschka (1953«) 

 have estimated the degree of polyploidy of certain cells in a tissue as 

 various multiples of the basic number, up to 256-ploid (Fig. 19-13). 

 Somatic polyploidy, as estimated either by direct count or by nuclear 

 size, commonly increases with distance from the apical meristem (Fig. 

 19-14) and with age (Fig. 19-15). 



All this obviously has a Very important bearing on the problem of 

 differentiation. Wipf and Cooper (1940) for example, found a close rela- 

 tion between the presence of naturally occurring tetraploid cells and the 



