40 Growth 



tion of the nucleus. The effect of dehydration in checking mitosis has also 

 been reported by Mole-Bajer (1951). 



Osmotic concentration of the cell sap was observed by Becker ( 1931 ) 

 to be inversely proportional to cell size and to number of chromosome 

 sets in polyploid moss protonemata. 



Various external factors are important both for the division and the 

 enlargement of cells. Light frequently tends to check division, and ultra- 

 violet radiation may inhibit it. The hypothetical mitogenetic rays of Gur- 

 witsch (1926) and his school were thought to stimulate mitosis. The effect 

 of light on cell size has been emphasized by Straub (1948). Most of the 

 elongation of etiolated plants (p. 309) is due to increase in cell length. 

 Giese (1947) has reviewed 300 papers dealing with the effects of various 

 kinds of radiation on the induction of nuclear division. 



Temperature, so important in many protoplasmic processes, has an 

 effect on mitosis. P. C. Bailey (1954) has shown that in Trillium the 

 maximum rate of cell division takes place at considerably lower tempera- 

 tures than does the maximum rate of increase in root length. Burstrom 

 (1956) reports that under higher temperatures the final cell length in 

 roots is less because of the shorter period of cell elongation. 



Wagner (1936) found evidence that gravity influences mitosis. In root 

 tips placed horizontally he observed that after about an hour there was a 

 marked increase of mitoses on the upper side, so that the very tip of the 

 root bent down. After 4 hours mitoses were equally distributed, and after 

 10 to 12 hours they were more abundant on the lower side and the tip 

 straightened out again. These changes were quite independent of the geo- 

 tropic bending due to auxin and cell expansion, which was evident much 

 farther back from the tip. When plants were grown on a clinostat. Brain 

 (1939) observed in lupine seedlings that cells of the cortex, endodermis, 

 and pith in the hypocotyl were larger than those of upright plants but 

 that in the radicle they were smaller. 



In some cases pressure stimulates division, as can be seen in the cortical 

 tissues through which a lateral root pushes its way ( Tschermak-Woess 

 and Dolezal, 1953). 



Water is of marked influence in determining cell size. The amount of it 

 available often determines how much a cell can expand (Thimann, 1951). 

 Zalenski and others (p. 325) have observed that at successively higher 

 levels on a plant the cells of the leaves are smaller, presumably because 

 of their inability to become fully turgid while they were expanding. Water 

 may have other effects. Funke (1937-1939) found that if some water 

 plants are put into deep water their petioles elongate rapidly by cell ex- 

 pansion, sometimes lengthening tenfold in 2 days. 



The role of the wall in cell growth has been much discussed. Does the 

 wall merely stretch under the pressure of an expanding vacuole or does 



