THE CYTOPLASM 



differ, and indeed they continue to show this difference after 

 thirteen years of vegetative propagation. 



The effect of the cytoplasm is maintained even through sexual 

 reproduction. Ordinary haploid gametophytcs obtained as segregates 

 from the hybrid sporocarp can be back-crossed as females to the 

 male parent. After several generations of such back-crossing, the 

 nucleus, of course, becomes effectively identical with that of the 

 male parent. Yet, after eight generations, Wettstein found that the 

 plants were still different from the male parent. The cytoplasmic 

 determinant was still unchanged, unimpaired by the continuous 

 government of an alien nucleus. In other words it has the properties 

 of a gene, a plasmagene. 



The plasmagene, or plasmagenes, in Funaria are unconditional 

 in their effect, so far as the experiment goes, but elsewhere the nucleus 

 also plays a part. The Fj cross between tall and procumbent flax, 

 Linum usitatissimum, is normal both ways. But, when the procumbent 

 variety is the female or cytoplasmic parent, one quarter of the Fg 

 is male-sterile ; the anthers abort. Here a nuclear gene of the tall 

 flax is reacting with a plasmagene of the procumbent. Or perhaps 

 it is failing to react because a plasmagene of the tall flax is missing 

 in the procumbent. No other combination gives this defect. And 

 it is constant over many generations back-crossed with pollen from 

 the tall. 



In Linum, just as with the plastids in Oenothera, the nucleus and 

 the extra-nuclear component in heredity fit one another in their 

 ancestral or habitual combinations, but fail to fit in some of the 

 combinations or re-combinations that can be produced by crossing. 

 The commonest symptom of this failure in the flowering plants 

 appears at the last and most delicate stage of development, the 

 formation of the pollen. Where, in Petunia and Nicotiana, self- 

 compatible and self-incompatible species (whose properties, as we 

 shall see, depend on pollen adjustment) are crossed, it is the nucleus 

 of the self-incompatible parent whose activity fails in the cytoplasm 

 of the self-compatible one. Thus it seems that the incompatibility 

 mechanism depends on the adjusted properties of the two com- 

 ponents, nucleus and cytoplasm. Changes in the cytoplasm can be 

 related to constructive adaptation even though they may not be 

 generally necessary for it. 



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