POLLEN GRAINS IN THE I'lU IT OF THE ASPEN 



925 



The number of pollen grains in a microgram is determined with a counting 

 chamber as used in the determination of red corpuscles. 



A suspension containing about 0.5 mgm pollen per ml was found to be best 

 suited for such determinations. 



A few thousand of pollen grains and a few hundred fruits sufficed for the 

 radioactive measurement. Such measurements were carried out partly by com- 



FiG. 1. Radioautograph ^f an aspen 

 leaf. The branch bearing the leaf was 

 placed in a solution containing ^^p 

 for 8 days. 



paring the activity of a known number of pollen grains and fruits and partly 

 by ashing a known weight of pollen or fruit and precipitating the phosphorus 

 in an ahquot as magnesium ammonium phosphate. The activities of the fractions 

 were then compared. 



RESULTS 



The first experiment was started on April 3rd, 1944. On April 19th 

 the grains were ready to be investigated. One pollen grain was found 

 to weigh 0.003 /^gm and to have an activity of 4.3 xlO"^ counts per 

 minute. One count per minute thus corresponds to 232 pollen grains. 



A seed was found to weigh 0.04 mgm and to produce 0.036 counts per 

 minute. One count per minute thus corresponds to 28 seeds. From these 

 figures it follows that one seed contains the labelled phosphorus originally 

 present in 8.3 pollen grains. A catkin investigated contained 537 seeds 

 and hence as much labelled phosphorus as was originally present in 

 4,450 pollen grains. 



The second experiment carried out in 1945 did not yield a sufficient 

 amount of labelled pollen. 



The third experiment was started on February 19th 194(3. The aspen 

 branch being kept for 7 days in an active solution. About 0.5 gm pollen 

 was obtained containing about 0.5 per cent of the activity added. The 



