It is possible, however, that the segregation of characters in the gametes may depend upon something far more subtle and elusive than the chromosomes or even of possible combinations of units within the chromosomes, but so far as we can see at present these are the only structures in the cell with which it can be satisfactorily associated.
In 1902, an American named Walter Sutton noticed that chromosomes duplicated themselves before cells divided so that each new cell had a full copy of the chromosomes.
Mottier, The Development of the Heterotype Chromosomes in Pollen Mother-cells, Ann.
Blackman, who also succeeded in showing that the nuclei of the sporophyte generation contain twice as many chromosomes as the nuclei of the gametophyte.
From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized fon purposes of division.
Eight chromosomes appear again in the ultimate divisions which give rise to the carpospores.
E, Chromosomes ox the spindle.
Further, it is only in the nucleus and in its chromosomes that we have any visible evidence to account for the Mendelian .segregation of characters in hybrids which are known to occur.
Other observers have in recent years demonstrated a similar relation in other genera between the number of chromosomes in the nuclei of the two generations.
The relation of the nucleolus to the chromosomes is clearly seen in the reconstruction of the daughter nuclei after division in the cells of the root-apex of Phaseolus (fig.