Vemalizadon and Photoperiodism — 68 — A Symposium 



that small fragments of embryos may also be vernalized. Vernalization of 

 embryos, therefore, is not dependent upon substances received from the 

 endosperm during germination. If substances of a hormone nature are 

 involved in vernalization, they may be manufactured in the embryo itself. 

 Purvis has shown, however, that whole grains of rye respond more rapidly 

 than do excised embryos and suggests that certain substances received 

 from the endosperm speed up vernalization. Cholodny (10) suggests 

 that these substances are auxins or the B vitamins or a certain combination 

 of them. 



As far as the author is aware, no one has successfully substituted for 

 the low temperature treatment the application of pure substances or ex- 

 tracts of plant material. The work of Melchers (23, 24) with Hyoscya- 

 mus presents encouraging possibilities in this direction, but more evidence 

 is necessary with other plants before one may be encouraged to postu- 

 late that a hormone may be found which, upon application to the plant, may 

 induce vernalization without a low temperature treatment. Lang and 

 Melchers (21) suggest that those plants which require vernalization 

 {i.e., a. low temperature treatment) do not make the flower-forming hor- 

 mone prior to vernalization. Subsequent to vernalization they may pro- 

 duce the flower- forming hormone provided environmental conditions are 

 favorable. Presumably such plants will flower without vernalization pro- 

 vided the hormone is supplied from some external source (i.e., from a 

 graft-partner). On such a basis, vernalization simply removes some 

 physiological restriction to the production of the flower-hormone. 



Theories Regarding the Mechanism of the Formation of a 

 Flowering Hormone: — Generalizations with respect to the possible 

 action of a flower-forming hormone are difficult because of the variations 

 in response which occur in different species of plants. Various investiga- 

 tors have attempted to correlate the responses of all long-day plants or 

 all short-day plants or all plants of the various classifications on the basis 

 of a given series of reactions of a similar nature. Some postulate the 

 presence of a single flower-forming substance of similar nature in all plants 

 and ascribe the variations in response to given environmental treatments 

 to the influence of these treatments in affecting the rate of synthesis of 

 certain precursors. Critical evaluation of certain theories must await 

 additional research with many different kinds of plants. A review of 

 some of the theories may be of interest from the standpoint of indicating 

 what additional information is necessary before any one theory is to be 

 favored or discarded. 



A recent paper by Lang and Melchers (21) gives a summary of 

 most of their work with Hyoscyamus nigcr. They have studied two forms 

 of this plant, one of which is an annual and the other a biennial. The 

 biennial form must be exposed to a certain period of low temperature 

 before it may be induced to flower. Subsequent to the low temperature 

 treatment, it apparently reacts in every particular in the same manner as 

 does the annual form. The annual form is a long-day plant. In ten-hour 

 days it remains vegetative, whereas it flowers when the days are over ten 

 hours and forty minutes. Removal of all of the leaves results in the 

 prompt initiation of floral primordia. On the other hand, if one of the 



