The Cellular Basis of Growth 43 



Moewus (1951), one for cell division in Protosiphon which is linked to 

 sex manifestation; and Nickerson and Chung (1954), one in yeast that 

 seems to block the sulphydryl mechanism of division. Dorries-Ruger 

 (1929) grew protonemata from the spores of plants produced by various 

 combinations of genome and plasmon in mosses, among races developed 

 in Wettstein's laboratory. She cut off and cultured the tip cells of these 

 protonemata and recorded marked differences in the rate of cell division 

 in the filaments growing from them, thus comparing the effects of dif- 

 ferent genotypes under the same environment. 



PLANE OF CELL DIVISION 



Cell size and cell number are important elements in growth and differ- 

 entiation, but the problem of form is primarily dependent not on these 

 factors but on the relative directions in which growth occurs. These, in 

 turn, are closely related to the planes of cell division in the developing 

 tissues. Whatever determines the position of the new cell wall between 

 two daughter cells will determine the direction in which these cells ex- 

 pand, since this direction will normally be at right angles to the new wall. 

 At the time of cell division, therefore, the direction of growth in this par- 

 ticular region of the meristem is determined. If plant cells could change 

 their relative positions, as is possible in many animal tissues, the deciding 

 factor in the direction of growth would often be cell movement rather 

 than plane of cell division. 



The position of the cell plate at telophase, and thus of the new cell wall, 

 follows the position which the equatorial plate of chromosomes finally 

 assumes at metaphase. The mitotic figure may roll around somewhat be- 

 fore it settles down to a permanent position, but there must be something 

 that determines that position. This raises the question as to whether the 

 plane of division is controlled by whatever decides the final orientation 

 of the mitotic figure or whether this, in turn, is itself determined by other 

 factors. That the latter may be the case is suggested by the way in which 

 vacuolate cells divide. In such cells a series of cytoplasmic strands forms 

 a loose diaphragm, the phragmosome, across the cell, and in the middle 

 of this the nucleus is supported. In tissues where the plane of division 

 can be predicted, observation shows that the position of this diaphragm 

 is the one which the future cell wall will occupy. The diaphragm is laid 

 down considerably before the nucleus enters metaphase, and the meta- 

 phase plate of chromosomes may not at first lie parallel to the diaphragm, 

 though it finally does. This seems to indicate that the plane of division is 

 determined early and for the cell as a whole rather than by factors acting 

 on the mitotic figure alone. There is also evidence that the mother cell, 

 before division, begins to elongate at right angles to the direction in 



