Criteria for Judging Polyploidy 369 



True autotetraploids have larger grains than the diploid (Fig. 

 15.1//, G). Microscopes are equipped with measuring oculars that 

 make this procedure routine. The correlation between the size ot 

 the pollen grain and the number of sets of chromosomes has been so 

 well established that no further discussion need be made on this 

 point. Triploid jiollen grains are notable for their irregular dimen- 

 sions and are useful in separating triploid and tetraploid plants on 

 a field scale basis. 



The mean diameters for the diploid and tetraploid watermelon 

 \aricties ^vere 57.3 and 67.5, respectively. The smaller grains in trip- 

 loids averaged 62.1 and the larger sizes, 67.5. Similar size com- 

 parisons have been made for the guard cells of epidermal cells. A 

 photomicrograph (Fig. \5.\E,F) gives a visual picture of the dif- 

 ferences between the larger tetraploid and smaller diploid. Also the 

 distribution of guard cells varies; the diploid cells are closer together 

 than the tetraploid. 



The relation between the size of pollen grains and guard cells of 

 a given plant are important for the reasons discussed in the previous 

 chapter under the subject of periclinal chimeras. If the pollen is tetra- 

 ploid and the guard cells are diploid, treatment with colchicine has 

 produced a chimera in which the deeper layer that produced the 

 pollen was made tetraploid and the outer layer remained diploid. A 

 reverse situation may occur. In these instances the guard cell would 

 show tetraploid characteristics and the pollen, diploid. The breeding 

 behavior of such a plant would be that of a diploid. Seed from this 

 plant would not lead to the expected tetraploid types, according to 

 information based on the guard cell sizes. Sometimes, a mixture of 

 diploid and tetraploid pollen exists in the same anther, or mixtures 

 of diploid and tetraploid guard cells appear on the same leaf. These 

 cases are a result of mixoploidy, a direct action of colchicine. 



In cross section the leaf of the diploid is not as thick as that of 

 the tetraploid. Usually extra layers of cells of the mesophyll are 

 present. 



Pollen mother cells undergoing meiosis are universally used for 

 counting chromosomes and determining the associations between 

 chromosomes during pairing. Acetocarmine stains have speeded up 

 such cytological work. Photomicrographs in Figure 15.1 show the 

 differences in numbers of chromosomes and some difference in the 

 association. Section D shows the multivalents in contrast to the one 

 in C (Fig. 15.1) .1" 



Other cells, such as the generative cells in pollen tube cultures, 

 root tips, and leaf cells, may be used for counting the number of 

 chromosomes. At the second meiotic division and the division of the 

 microspore, chromosomal counting may be easier than at the first 

 meiotic metaphase. 



