THE MODIFICATIONS OF THE CHROMOSOME CYCLE 55 



generations, either the haploid phase or the diploid phase but not both. 

 This is not true in higher plants and exceptions may be expected in the 

 comparatively few organisms (some algae) in which the haploid and 

 diploid phases are nearly identical in appearance. 



2. Haploid Heredity of Higher Plants 



In the higher plants the haploid phase is considerably reduced. In 

 the male the microspore forms only the pollen tube, in which one or 

 two nuclear divisions, unaccompanied by cell division, may occur, and 

 in the female the macrospore undergoes a variable number of divisions 

 giving some hundreds of nuclei in gymnosperms, but usually only 

 about eight nuclei in angiosperms. In spite of its short duration, even 

 the gametophyte of higher plants seems to be under the control of its 

 own special set of genetic factors. Most of the factors which have been 

 discovered affect the male. Two of the clearest examples are factors 

 which affect the reserved carbohydrate of the pollen grain. Pamell^ 

 showed that two races of rice exist, differing in a factor of which the 

 dominant allelomorph Gl produces starchy pollen which stains blue 

 in iodine, the recessive allelomorph gl a glutinous carbohydrate which 

 stains brown. An exactly analogous factor is known in maize, where the 

 waxy gene (recessive) gives a pollen grain containing a carbohydrate 

 which stains red in iodine. ^ In both these cases, hybrid plants form 

 equal numbers of the two types of pollen. 



Many factors have been found which affect the growth of pollen- 

 tubes. The effect may sometimes be a direct influence on the pollen; 

 in extreme cases factors are known which make it impossible for the 

 pollen to function (pollen lethals), as it seems to be in the well-known 

 case of Matthiola,^ where, however, this may be due to a chromosomal 

 deficiency.* 



The effect may be less extreme, merely causing one type of pollen 

 grain to grow faster than another.^ This phenomenon, so called certa- 

 tion, leads to aberrant ratios when a heterozygous plant is used as the 

 male parent. An example may be given from the work of Correns on 

 Melandrium in which it was shown that the female determining pollen 

 tubes grow faster and achieve fertilization more often than the male- 

 determining.^ 



Often, however, the expression of the factor depends on an inter- 



^ Pamell 1921. ^ Rev. Brink 1929. 



^ Saunders 1928, Waddington 1929. 



* Philp and Huskins 1931, but see Kuhn 1938. ^ Rev. Jones 1928. 



* Correns 1921. 



