Hamner — 67 — Hormones 



while the Sampson variety seems to be indeterminate according to Garner 

 and Allard's classification. When both are grown under long-day con- 

 ditions, the former remains vegetative while the latter produces flowers. 

 The experiments were conducted under continuous illumination, and vari- 

 ous types of grafts were made when the plants were at an age at which 

 the Sampson variety was ready to initiate flower buds. Decapitated plants 

 of Maryland Mammoth tobacco were partially defoliated, and scions of 

 either the Sampson or Maryland Mammoth variety grafted to them. Un- 

 der these conditions, the stock soon produced lateral branches, and these 

 flowered provided the scion was of the Sampson variety and was not de- 

 foliated. The stocks did not flower when Maryland Mammoth scions 

 were used. Kuijper and Wiersum (20) at about the same time obtained 

 analogous results with soybeans. Since that time many investigators have 

 found it possible to stimulate short-day plants to flower under long-day 

 conditions by grafting to them other plants of the same variety which have 

 been induced to flower by exposure to short day or by grafting to them 

 other varieties or species which will flower in spite of long-day conditions. 

 Most of the grafting experiments have been carried out using short-day 

 plants growing under long-day conditions as the test object and receptor. 

 MosKOV (26) obtained transfer of the stimulus from scions even 

 though the graft union between stock and scion was notably weak. Ham- 

 ner and Bonner (16) obtained transmission of the stimulus across what 

 they called "a diffusion contact." They separated the injured surfaces of 

 Xanthimn plants with lens paper and obtained transfer of the stimulus 

 from a plant treated with short day to the receptor plant maintained on 

 long day. They found no evidence of tissue contact between the injured 

 surfaces of the two plants and assumed that only substances capable of 

 diffusion would have been transferred from one plant to the other. With- 

 Row (38) repeated these experiments and found that in those cases where 

 transmission of the stimulus occurred cells had grown through the lens 

 paper from one plant to another although admittedly the union between 

 the two plants was very slight. 



Vernalization: — A detailed discussion of vernalization has been given 

 in another chapter. For purposes of discussion here, vernalization will be 

 considered to involve the treatment of certain plants with low tempera- 

 ture in order to induce subsequent flowering. Many plants fail to flower 

 unless exposed to a certain period of low temperature either in the seedling 

 stage or after they have developed a number of foliage leaves. 



There is a little evidence that hormones may be involved in the vernali- 

 zation process. Melchers (23, 24) induced the biennial form of Hyos- 

 cyamus niger to flower without a cold treatment by grafting to it the 

 flowering plants of Hyoscyamus albus, Petunia hybrida, and Nicotiana 

 tabacum. These results indicate that this particular plant, which is ordi- 

 narily assumed to require vernaHzation, may be induced to flower as the 

 result of a stimulus received from other flowering plants without the neces- 

 sity of vernalization treatment. There is little additional evidence with 

 other plants of this nature. 



Purvis (36) has shown that excised embryos of rye grown on a cul- 

 ture medium may undergo vernalization. There is some evidence (35) 



