CH. VIl] FROST. 163 



or two, so that it may begin to wither. Impale the seed 

 on a pin transversely to the plane of the cotyledons, and 

 fix the pin in a clamp so that the root is horizontal and 

 1 — 2 mm. above a surface of water. Then gently bend the 

 root downwards till its lower surface and the water meet. 

 In a few minutes the tip of the root will be seen to 

 be rearing itself into the air, which is due to the increased 

 turgescence of the lower surface of the root. 



The same result may be obtained to a still more 

 marked degree if the roots are rendered flaccid by 

 immersion in 5°/o NaCl solution before being placed 

 in contact with the surface of water. 



(209) Drooping of leaves during afrost^. 



In connection with exp. 198 it is of interest to note 

 that flaccidity may be a cause of curvature. The leaves 

 of the laurel, Prunus laurocerasus, and of the Portugal 

 laurel, P. lusitanica, droop in a striking way during sharp 

 frost. If a branch is brought into a warm room the leaves 

 rapidly assume a normal position. The droop is clearly 

 due to a sharp curvature in the petiole : if leaves are cut 

 and placed in lukewarm water the straightening of the 

 petiole occurs at once and is clearly visible to the naked 

 eye. Fig. 32 shows a laurel twig, A in the frozen, and B in 

 the thawed condition. It seems most probable that these 

 movements are due to flaccidity, because if a branch 

 is fixed so that its axis points vertically downwards the 

 leaves still sink during frost ; in this case the sharp curve of 



^ Moll, Archives Neerlandaises, T. xv. p. 13 of separate copy. 



11—2 



