302 INTRODUCTION TO CYTOLOGY 



(x). Consequently the female parent contributes 3C chromosomes (24 in 

 one polar nucleus and 12 in the other) to the endosperm, while the male 

 parent contributes only 12; thus the endosperm has 48 (4x) chromo- 

 somes instead of the normal 36. 



Although in the great majority of known examples endosperm is 

 formed by the repeated division of a triple fusion nucleus, cases are known 

 in which it is produced by the polar fusion nucleus (embryo sac nucleus) 

 alone without the male, or by the fusion product of the male and one 

 polar, or by one polar alone. Combinations of these three methods may 

 be found in the same embryo sac. The development of the endosperm 

 may be initiated by the formation of a number of free nuclei which are 

 parietally placed and in later mitoses become separated by walls, or 

 by the formation of walled cells from the start. (See Coulter and Cham- 

 berlain, 1903.) 



The term xenia was applied by Focke (1881) to the effect of foreign 

 pollen on the endosperm of the resulting seed in angiosperms. Thus if 

 maize of a certain strain which produces seeds with white endosperm when 

 self-pollinated, is pollinated with pollen from a plant whose seeds have 

 red endosperm, the endosperm of the resulting hybrid seeds is red like 

 that of the pollen parent. No satisfactory explanation of this phenom- 

 enon was at hand until the discovery of double fertilization by Nawas- 

 chin and Guignard in 1898-9. It then became clear that the endosperm, 

 which was formerly supposed to contain only maternal nuclear material, 

 may show endosperm characters of the parent furnishing the pollen for the 

 reason that the latter contributes a nucleus to the primary endosperm 

 nucleus, so that every endosperm cell contains some nuclear material 

 from the pollen parent. 



Normally the endosperm cells are all alike in containing two chromo- 

 some sets from the female parent and one set from the male, and the nor- 

 mal inheritance of endosperm characters as well as all ordinary cases of 

 xenia can be understood on this basis. Mottled or mosaic effects in the 

 endosperm of maize hybrids were attributed by Webber (1900) to such 

 abnormal modes of endosperm origin as were referred to in a foregoing 

 paragraph: some of the cells may have been formed by polar nuclei 

 of purely maternal constitution while other cells were the result of the 

 independent division of the second male nucleus. Although this explana- 

 tion may fit some cases, it is becoming apparent from the work of Emerson 

 and others that most of them can be better accounted for on the basis of 

 aberrant chromosome behavior, such behavior having been observed in 

 certain other organisms. 



Additional evidence is found in all these phenomena for the theory 

 that the nuclear substance in some way represents the physical basis of 

 inheritance. The second male nucleus not only is concerned in the 

 initiation of the development of the endosperm, but xenia shows that it 



