DEPARTMENT OF GENETICS. 



95 



chromosomes each and 1 set of 6 homologous chromosomes, gave the following 

 in its selfed progeny: 



Numbers of chromosomes 48 49 50 51 52 



Numbers of plants 1 9 10 1 



Calculated, 24-chromosome pollen only 3 16 3 



Calculated, all pollen functional 4 4 13 0.3 0.3 



"The results agree with the view that some pollen with extra chromosomes is 

 functional here." 



Diploid Plants with One Extra Chromosome. 



Investigation ad hoc is now being made of the chromosomes of the plants 

 with 25 of them. The following methods are being used: (1) Camera draw- 

 ings are made of several trivalents from each kind of plant; with a view to 

 obtain accurate size measurements and determine the relative frequencies of 

 different methods of combination of the 3 chromosomes; (2) camera drawings 

 are made of complete groups of chromosomes at the first metaphase, to 

 determine with certainty the different sizes; (3) size estimations are taken 

 at the second metaphase, among the group of 13 chromosomes; (4) observa- 

 tion is made of the size of the detached chromosomes found in the 12+1 + 12 

 distributions. 



Diploid Plants with One Chromosome Missing. 



Portions of two plants and one whole plant were found by Doctors Blakeslee 

 and Belling in this condition. Apparently in two (or in all three) of the 

 cases it was the largest chromosome which was missing. There were 11 

 bivalents and 1 univalent at the late prophase and first metaphase. At 

 the second metaphase the unpaired chromosome appeared larger than in 

 the regular diploid. 



Tetraploidy and Evolution. 



If in any species the chromosome group in the soma is divisible into sets 

 of 4 chromosomes of each size, it is probable that the species is either a 

 tetraploid or has come from tetraploid ancestry. It has been found that 



1 2 



Fig. 1. — Pollen-grain of Uvularia grandiflora, showing 7 chromosomes, of 6 different sizes. 

 Fig. 2. — Pollen-grain of hyacinth (Yellowhammer), showing pairs of similar chromosomes. 



often, in monocotyledons especially, an estimation of the sizes of the chromo- 

 somes is best made in the pollen-grains themselves at their first division, 

 instead of in the pollen mother-cells or in the root tips. Plants which 



