364 



SCIENCE 



[N. S. Vol. L. No. 1294 



the leaves are separated from the parent plant 

 and placed in water or in a saturated atmos- 

 phere. We have found that when a length 

 of 2 to 3 centimeters of the petiole is kept at 

 a temperature 2.5° to 3° and the leaf immersed 

 in water at room temperature, the notches, or 

 some of them, will develop into new plants, 

 while the leaf is still attached to the parent 

 plant. The low temperature does not visibly 

 injure the petiole and after its removal the 

 development of the notches may again be in- 

 hibited or retarded. Moreover, it can be 

 shown in this way that each leaf exerts a cor- 

 relative action upon the leaf opposite and to 

 some extent upon other leaves near it, this 

 action extending farther down, the stem than 

 upward. For example when the petiole of one 

 leaf is kept at low temperature, development 

 occurs not only in the notches of this leaf 

 but in at least some of those of the opposite 

 leaf and sometimes in some of the notches of 

 the next pair above and of one or two pairs 

 below, if these leaves are immersed in water. 

 In general the effect upon other leaves of the 

 physiological isolation of one leaf apparently 

 decreases with increasing distance from the 

 isolated leaf and more rapidly upward than 

 downward. 



Very commonly the development of the 

 notches is less rapid in the leaf with the low 

 temperature zone on its petiole than in the 

 opposite leaf, although the two leaves them- 

 selves are at the same temperature. This 

 difference in rate of development is probably 

 due to the fact that the low temperature zone 

 brings about some disturbance in the move- 

 ment of nutrition to the leaf and so delays 

 somewhat the development of its buds. 



Young plants of the scarlet runner bean 

 Phaseolus multiflorus, and Lima bean, Pliase- 

 olus niacrocarpus have also been much used in 

 these experiments with even more satisfactory 

 results than Bryophyllum. In these species 

 buds are present in the axils of the cotyledons, 

 but in normal plants these buds never develop 

 beyond minute outgrowths. When a length 

 of 2 or 3 centimeters of the stem above the 

 cotyledons and below the first pair of leaves is 

 inclosed in a coil at 3°, 4° and usually even 5° 

 C, these buds develop, although the plant 



above the zone of low temperature shows no 

 wilting and at most only a slight retardation 

 of growth for two or three days. Moreover, 

 the development of all axillary buds above the 

 zone of low temperature is inhibited as in nor- 

 mal plants, but the inhibiting factor is blocked 

 by the low temperature zone, although water 

 and nutritive substances obviously pass it. 

 When the low temperature coil is removed 

 from the stem the growth of the buds in the 

 axils of the cotyledons may be again inhibited, 

 and may be again started by replacing the low 

 temperatiu'e. If, however, the physiological 

 isolation by low temperature is maintained 

 long enough to permit these buds to develop 

 into shoots several centimeters long, they 

 often continue to grow more or less rapidly 

 after the low temperature is removed and in 

 some cases may even inhibit the further 

 growth of the chief tip. In these bean seed- 

 lings the effect of the low temperatiu'e zone 

 is visible in the growth of the buds within 

 one to two days. 



In the same way other axillary buds at 

 higher levels of the stem may be physiolog- 

 ically isolated and induced to develop, while 

 the chief growing tip continues to grow and 

 to inhibit all buds above the zone of low 

 temperature. In most plants a temperature of 

 5° or 6° C, constitutes an effective block 

 to the inhibiting action of the chief growing 

 tip for several days, but these temperatures 

 are near the upper limit of effectiveness for 

 this species, and adjustment or acclimation of 

 the cooled zone gradually occurs to such a 

 degree that the buds which were at first phys- 

 iologically isolated are again inhibited and 

 cease to grow even before the low temperature 

 is removed. When lower temperatures are 

 used such acclimation may occur to some ex- 

 tent, but is less rapid. 



Physiological isolation of the runner-tip of 

 Saxifraga sarmentosa may also be brought 

 about by low temperatiire. The runners of 

 this plant, like those of the strawberry and 

 various other forms, grow to a certain length, 

 and then the bud at the tip of the runner 

 develops into a new plant, and growth in 

 length of the runner ceases. The more active 

 the parent plant, the longer the runner grows 



