R. P. Gregory 317 



they tend, I think, not so much to call in question that aspect of the 

 theory with which we are concerned, as to support the view that the 

 plastid-origins arc handed on from parent to offspring as definite 

 bodies. 



Granting that the plastids are persistent cell-organs, the existence 

 of mixtures tif plastids of different kinds in certain cells renders it 

 difficult to resist the inference that the abnormality lies in the plastid 

 itself and not in the surrounding cytoplasm. The inference is supported, 

 I think, by the fact that different degrees of chlorosis may occur in the 

 same plant, and by the fact that some entirely chlorotic plants have 

 been found to be mosaics of cells, the plastids of which exhibit well- 

 marked differences in the degree of chlorosis. 



The hypothesis that, in the higher plants, the plastids of the s^ygotc 

 are genetically derived fi-om those present in the unfertilized egg-cell 

 has, like the general theory of which it forms an extension, been widely 

 accepted ; but it is obvious that it remains an assumption for any 

 particular species, until that species has been the subject of special 

 investigation. In any case, however, the assumption that no plastids 

 (or plastid-origins) pass over in fertilization, from the male to the 

 female gamete, demands less than does the alternative assumption that 

 no cytoplasmic structures of any kind accompany the male nucleus into 

 the egg-cell. In many cases the nucleus of the male cell has been 

 described as becoming disengaged from the cytoplasm, which does not 

 enter the egg-cell ; on the other hand, there are cases among the 

 higher plants, in which the male generative cell as a whole enters 

 the egg, and, in some cases, bodies resembling leucoplasts have been 

 observed in the mass of cytoplasm brought into the egg with the male 

 cell'. This may, however, be compared with the behaviour of the 

 chloroplasts in certain species of the alga Spirogyra. In this genus, 

 the two gametes contribute almost equally to the cytoplasm of the 

 zygote, the chloroplast (oi- chloroplasts) of the male gamete passing 

 into the zygote along with the other structures of the male cell ; but 

 Chmielewsky- and Trondle^ have shewn that the chloroplast of the 

 male gamete degenerates after entering the zygote, while that of 

 the female gamete alone persists and becomes the chloroplast of 

 the zygote. 



" For instance in Pinus. See V. H. Blackman, Phil. Trans. Roy. Soc. B. Vol. 190, 

 1898, pp. 395—426. 



2 Bot. Zeit., Jahrg. xlviii. pp. 773—789, 1890. 



» IM. Zeit. Jahrg. lxv. pp. 187—216, 1907; Zeitschr.f. Hot. iii. pp. .593—019, 1911. 



