212 SECTIONAL ADDRESSES. 



on the corolla segments have also been developed. With regard to 

 doubling, it is of interest to notice that in P. malacoides and in P. effusa 

 this has taken place within some four years after their introduction. 

 Nuclear changes to the tetraploid condition have been found in some of 

 these ' improved ' forms, notably in P. sinensis and P. malacoides, while 

 P. ohconica has also become tetraploid, but there is no obvious difference 

 between diploid and tetraploid plants.'^' If we invoke ' mutation ' we do 

 not seem to have advanced much further along the road, nor does intra- 

 specific hybridisation throw much light as to the commencement of varia- 

 tion, though it may be effective when we have reached some well-marked 

 varietal forms. It has been suggested that the effect of cultivation — i.e. 

 good living and good feeding — has in some way broken down the constitu- 

 tion of the plant and given rise to a tendency to vary, but we have no 

 definite evidence in support of this view ; in all such cases we should like 

 to know whether such variations occur among the plants as they grow 

 wild and which may have escaped observation. If this is so, as those 

 most competent to judge consider probable, we must assume that the 

 observation of variants under cultivation is due to the fact that horti- 

 culturists are always on the look-out for small varietal differences, which 

 as soon as they are noticed are selected and encouraged. 



The explanation of this tendency to vary, displayed by plants whether 

 under cultivation or in nature, should possibly be sought in the realms of 

 chemistry rather than of cytology — we have seen something of the remark- 

 able capabilities of the plant cell as a chemical laboratory in the case of 

 Eucalyptus dives and other plants, and it may be that very small additions 

 to, or deviations from, the normal food supply of the plant, as we know, 

 for instance, in connection with the researches carried out with boron and 

 manganese, or with our own transplant experiments with Plantago major, 

 may so disturb the composition of the cytoplasm that the whole internal 

 economy of the plant is upset and it becomes ' jilastic' As a result, since 

 the germ-plasm also is affected by the stimulus, the observant cultivator 

 is able to take full advantage of his opj^ortunity, and by careful selection 

 can develop and encourage the production of new and distinct forms. 



We now know from Prof. Goodspeed's recent work on the effect of 

 X-rays on the sexual cells of Nicotiana ^^ that these rays bring about some 

 striking changes in the germ-plasm, and we look forward presently to 

 hearing some further particulars from him on his interesting work. 



In all these matters we are still largely in the land of theory, but I 

 think we may say that in our strivings towards the truth we are catching 

 here and there flashes, which encourage us to proceed in our search for 

 the light which will enlighten our darkness. 



^' This has been worked out by Mr. Philp at the John Innes Horticultural] 

 Institution. | 



'^- See Goodspeed, Prof. T. H., ' The Effect of X-rays and Radium on species of | 

 the genus Nicotiana,' in Journ. of Heredity, Vol. xx, No. 6, June 1929, pp. 243-259 ; , 

 ' Cytological and other features of variant plants produced from X-rayed sex cells of 

 Nicotiana tabacum,'' Bot. Gaz. Ixxxvii, No. 5, June 1929, p. 563, and ' Occurrence of i 

 Triploid and Tetrapoid Individuals in X-ray Progenies of Nicotiana tabacum,' in Univ. 

 Calif. Publ. i., Botany, Vol. ii, No. 17, pp. 299-308, 1930. See also MuUer, H. J. ' The 

 Problem of Genie Modification,' Zeitschr. fiir Indukt. Abstam. und Vererb., Supple- 

 ment-band I, 1928, p. 234. 



