Vernalization and Photoperiodism —10— A Symposium 



action during germination, in the early stages of which the phytohormones 

 are transmitted to the organs of the embryo which are beginning their 

 growth and development. Schander showed that the activating substances 

 contained in cereal grains at the very beginning of germination pass over 

 to the embryo through the special elongated cells of the aleurone layer, and 

 regarded these substances as being required for further development and 

 growth of the embryo. Cholodny (1935) claimed that auxin or a related 

 compound which is accumulated in the endosperm during germination of 

 cereal grains is also transmitted into the embryo. This auxin has been 

 identified as indole-3-acetic acid (Haagen-Smit, Leech and Bergren, 

 1942) ; it is rapidly liberated from its storage form in the endosperm by 

 simple moistening or alkali hydrolysis, differing in this respect from the 

 auxin in leaves and stems (Thimann and Skoog, 1940). 



In 1936 Cholodny formulated the nature of the internal factors which 

 act upon the growing points during vernalization. According to the hypo- 

 thesis which he developed, the embryo is stimulated to activity by the pre- 

 treatment technique but lacks the capacity for normal growth, because of the 

 insufficient moisture and low temperature accompanying the vernalization 

 treatment ; the embryo absorbs from the endosperm the growth hormones 

 which are stored there in great quantity. As these hormones are generally 

 used primarily for growth, and as under the conditions of the vernalization 

 treatment there is almost no growth, the concentration of the hormone in 

 the cells of the embryo is assumed to have risen considerably above the 

 normal for that stage of the plant's life. It is further assumed that this in- 

 crease of the intracellular concentration of the hormone in the growing points 

 causes an acceleration in the progress of the meristematic cells of the young 

 plants through the first phases of development. Thus the interval of time 

 which separates these first phases of development from the later stages asso- 

 ciated with preparation for fruiting is shortened. The full result is assumed 

 to be that the whole cycle of plant development is completed sooner than it 

 would be under normal conditions (Cholodny, 1939). 



Reference will be made in a later section to the experiments of Gregory 

 and Purvis on the vernalization of excised embryos, on which they base their 

 own criticism of Cholodny's hypothesis. Cholodny felt the need to intro- 

 duce a correction into his hypothesis because of the rapid advances being 

 made in the knowledge of phytohormones. "It is hardly possible at the 

 present time to doubt that the qualitative aspect of the complex of phytohor- 

 mones acting upon the embryo is not also without its own effect upon the 

 development of the embryo. The composition of that complex and the 

 chemical nature of the active substances found in the tissues of the embryo 

 and of the plant developing from it undoubtedly also change under the in- 

 fluence of the external and internal conditions in which development is 

 being maintained." 



Cholodny considers that his working hypothesis is confirmed by some 

 of his own experiments on the pre-treatment of seed with various solutions 

 of indole-acetic acid, and blastanin or embryo extract (Cholodny, 1936), 

 as well as by some later experiments by Thimann and Lane (1938) and 

 by Cailahjan and Zdanova (1938). These investigators found that a 

 short treatment of growing seeds with increased concentrations of phy- 



