26 DURATION OF THE SEVERAL MITOTIC STAGES 



versely, and all are about the same size. Thus the cell number, on 

 the average, is roughly proportional to root-tip length in this actively 

 growing tissue. 



The onions used in these experiments were uniform in size and exter- 

 nal appearance and, while they were purchased in the open vegetable 

 market without their pedigree being known, they were of sufficiently 

 uniform type and sprouted with sufficient uniformity to convince one 

 that their genotypic constitution was quite uniform. An effort was 

 made to divide a single onion into 3 equal vertical sections and to sprout 

 the roots from each section under the 3 different but constant tempera- 

 tures, thus eliminating a possible genotypic difference. It was found, 

 however, that there were not enough root-tips of uniform size in each 

 section to supply the demands of the study, 57 being required. Five 

 onions were grown in each temperature-constant chamber. The 19 

 samples required for each temperature-series were cut from these five 

 onions on the basis of uniform length and appearance. 



APPARATUS: THERMOSTAT. 



Constant temperatures in growing conditions were required and, 

 in the absence of laboratory rooms with equipment especially designed 

 for maintaining constant temperature, a special apparatus had to be 

 built. This consisted of a battery of 3 constant-temperature boxes, 

 each 1 foot by 1 foot by 1| feet in size, mounted longitudinally about 

 a foot apart upon a board. Each box had a wooden top, bottom, and 

 ends, but the front and back were inclosed with double glass doors. 

 Underneath these chambers ran a wooden tunnel, heated at the extreme 

 right with a small kerosene lamp. Since the CO2 contents of the 3 

 chambers must be constant, the fumes from the lamp were not allowed 

 to enter the tunnel, which was separated from the lamp-container by a 

 zinc partition. Aloxig the top of the chambers ran a similar tunnel, 

 connecting from above with a well-insulated ice-box in which the cool- 

 ing substance (crushed ice and salt) was confined to three-fourths of the 

 space (left-hand) by a wire netting. From each tunnel into each box was 

 an opening covered by a small copper lid slightly controlled by thermo- 

 stats taken from Hoover incubators. The lids and thermostats were 

 so adjusted that a rise in temperature lowered the lid which covered the 

 warm-air opening, and uncovered further the opening from the cold- 

 air tunnel. When the temperature fell, the reverse action was induced. 

 A centigrade thermometer was inserted through a cork which filled a 

 hole in the top of each chamber; the thermometer was long enough to 

 extend into the water in which the onions grew. In each tunnel on 

 each side of each box were hand-dampers controlling the size of the 

 tunnel. It must be confessed that, even at best, this contrivance was 

 was only partially automatic. In order to keep the temperature of each 

 compartment within the range of 1° C. from the desired standard, it 



