Chapin: Heredity in Chimeras 



545 



early stage in the growtli of the embryo 

 all the chloroplasts in each cell will be 

 descended from a single mother chlQro- 

 plast, white or green. 



The second difficulty Baur answers 

 by saying that the white and the green 

 seedlings are probably extreme types 

 of chimeras in which the apparently 

 missing green or white tissue is repre- 

 sented by only a few cells which escape 

 notice with the naked eye. He says he 

 has been able to find small islands of 

 white or green cells in seemingly pure 

 green or pure white seedlings, though 

 whether they are present in all cases he 

 is, of course, unable to say. 



According to this theory we would 

 expect that the extreme types, the all 

 green and the all white seedlings, would 

 be rather scarce as compared with the 

 variegated plants, instead of the heavy 

 majority of green seedlings which Baur's 

 figures show. In my own crosses 

 variegated plants have always been in 

 the minority, though in one case the 

 white plants outnumbered the green. 

 This is doubtless due to some unknown 

 factors in the mechanism of develop- 

 ment. 



A good start has been made in the 

 study of chlorophyll inheritance, but 

 much remains to be done. Several of 

 the theories given rest on the shaky 

 foundation of few observations and 

 small numbers. A discovery is hardly a 

 discovery until others have repeated 

 the work with the same results. The 

 great importance of this work is seen 

 when we remember that the welfare of 

 all the higher forms of life depends 

 ultimately on the work of the chloro- 

 plasts. 



SUMMARY. 



A pigweed, Amaranthus retrojiexus, 

 was found having variegated leaves. 

 The plant was evidently a sectorial 

 chimera composed of green and white 

 tissue. Its offspring consisted of green, 

 white and a few variegated seedlings 

 like the mother plant. Green seedlings 



1 Ernst A. Bessey writes me: "The probability that the pollen carries over any anlage for 

 chloroplasts is very slim, as it is only the two male nuclei from the pollen tube that seem to be 

 essential, and only one of them enters the egg. These nuclei slip out of the cytoplasmic sheath 

 way up in the style so that they simply enter into the fertilization as naked nuclei and not as in 

 the lower plants, as whole cells. In some plants chloroplasts have been observed in the egg cell 

 and in most plants where it has been investigated the anlage of such chloroplasts are present." 



apical cone is made up of two sorts of 

 cells, one of which produces green 

 tissue and the other only white. In the 

 sectorial chimeras pure green or pure 

 white branches are frequently fonned 

 from buds starting in green or white 

 tissue. Periclinal branches arise from 

 the overgrowth of one tissue about the 

 other. 



The heredity of these plants involves 

 two difficulties: 



1. Why the union of green and white 

 germ cells does not produce Mendelian 

 heterozygotes. 



2. Why such a cross should sometimes 

 produce variegated chimeras and at 

 other times give rise to green or white 

 plants. 



Baur's answer to the first question 

 rests on the fact that the chloroplasts 

 have a heredity independent of the cells 

 in which they live. Chloroplasts orig- 

 inate from the division of other chloro- 

 plasts and in no other way. This does 

 not mean that mature chloroplasts are 

 to be found in embryonic or sexual cells, 

 but only that small bodies which develop 

 into chloroplasts are there and that 

 these small mother chloroplasts are not 

 produced from the cytoplasm or chro- 

 matin, as was formerly thought. ^ 



In the previous cases of abnormal 

 chlorophyll development which we have 

 studied we must suppose that the nor- 

 mal production of green leaves is pre- 

 vented by the absence of useful factors 

 or, in the case of the "albomaculata" 

 plants, by the presence of inhibiting 

 agents, acting on the chloroplasts. In 

 the white tissue of the chimeras, how- 

 ever, the chloroplasts themselves are 

 abnormal and will not properly develop. 

 The union of white and green gametes 

 results, then, in a zygote which contains 

 at least one white and one green mother 

 chloroplast. These divide into daughter 

 chloroplasts which are distributed 

 among the dividing cells at random. 

 Baur assumes that cell division at first 

 takes place more rapidly than chloro- 

 plast division so that eventually at an 



