Effects of Periods of Warm Weather upon the Winter Hardened Condition of a Plant.

The influence of alterniatinig periods of high and low temperatures upon the hardeninig process of plants has been described at various times in the literature (1, 2, 7). The rate at which planits lose this hardened condition when exposed to warm temiiperatures has been investigated (3, 10, 11). There appears to be relatively little in the literature, however, concernilng the ability of a plant to become hardy a seconid time, following the dehardening, process. Most of the work that relates to this subject is intimately conniected with the process of vernialization. The implication from-n the general theory of vernalization, as stated by LYSENKO, is that, since "every plant passes through a sequeniee of phases, which replace one another consecutively and in strict rotationi,' a winter-anlimual plant, such as winterwheat, would pass ilnto the hardeined conditioni once, and that, having passed out, it could searcely reenter that coniditioni. Sincie this inivestigation was be(gun, a number of papers have appeared that deal with this topic indirectly. Amonig these are KOSTJUCENKO anid ZARIJBAILO (8, 9), who have reviewed literature dealing with the subject of the dependeniee of winterhardiness on the coniditions of seed ripeninig. If the seeds of rye or wheat were vernalized Uponi the mother plant durinig a cold season of ripeninig, plants grown from theiii were founid lackincg in hardiniess. Thus, wheat grown in north Russia was founid to produce seed that was not suited for planting in that region. since it had been vernialized uponi the mother planit, whereas seed produced oni plants from the same lot of seed, but growni further south, produced winter hardy plants. GREGORY an1d PURVIS (5, 16) have presenited further evideilce along the samiie linie. RUDORF (12) subjected w%vinter cereals that had beeni vernalized to varying degrees, to a temiiperature of 100 C., with various day lengths for various periods of time, anid determlined the rate of loss of winiterhardiness and the degree to which these plants could be rehardenied. He conieluded that loss of the ability to retain hardiness or to rehardein was directly correlated with the deoree of vernalization anid with the developmnent of the growing poiiit. Voss (14, 15) has presented extensive evidenee along the same linles, showing that the degree of vernializationi, measured both by microscopic examination of the development of the growing poinit, and by subsequent growth behavior, is intimately related to the response of the planit to temiiperatures that ordinarily produce the hardened condition. The length of 1 Joulnal Article, new series, no. 440.