492 BULLETIN 428, U. S. DEPARTMENT OF AGRICULTURE. 
Among the factors influencing winterkilling may be mentioned low 
temperature, excessive variation in temperature, protection as 
afforded by snow and drifted soil, thickness of stand of plants, pre- 
cipitation, vigor of plants upon entering the winter, and condition of 
soil with respect to moisture, surface drainage, and existence of ice 
sheets. 
Medicago falcata is generally regarded as a hardy species, and it 
was for this reason that efforts were made to introduce as many of its 
forms as possible from regions where it is found growing naturally. 
While it scarcely could be expected that forms of the species from the 
Mediterranean region would be as hardy as those from the vicinity 
of Yakutsk, 62° north latitude, where the temperature reaches —84° 
F., it is probable that most of the forms can be placed in the class of 
comparatively hardy alfalfas, regardless of the geographical location 
in which they were developed. Just why some varieties of alfalfa 
are more susceptible than others to winter conditions is not clearly 
understood. It is now commonly believed, however, that an impor- 
tant factor in the hardiness of any variety is the degree of protection 
which the plant provides its dormant or resting buds. For example, 
varieties in which the crowns are produced well above the surface of 
the ground are uniformly tender, while those in which the crowns are 
near or beneath the surface are mostly hardy. The Arabian and 
Peruvian varieties may be cited as illustrations of the former, while 
the Grimm and Baltic varieties represent the latter group. The ~ 
dormant buds of the varieties with high crowns are fully exposed to 
the unfavorable conditions of winter, while those of the varieties 
having low crowns are protected by the soil, and to some extent by 
the dead herbage of the preceding summer. The relation of deeply 
set crowns to hardiness probably was first called to attention in 
agricultural literature by Thomas Le Blane in 1791 (32), but it is 
_ only within recent years that the significance of this characteristic of 
the crown has been duly appreciated (44, p. 4). 
A majority of the forms of I/edicago falcata produce at least a 
part of their new growth from underground members, either rhizomes 
or true lateral roots, a characteristic which affords material protec- 
tion during periods of severe conditions. It is largely upon this 
feature, as well as upon its geographical range, that the estimate of 
its hardiness has been based. | | 
While comparative tests of the hardiness of Medicago falcata and 
strains of I/edicago sativa grown in rows and hills have been made 
since 1910, there are still insufficient critical data from which to draw 
definite conclusions as to the relative hardiness of the former under 
actual field conditions. Unfortunately, many of the plantings that 
were made in 1909 were poorly provided with checks and did not 
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