The water in the pitcher-plant leaves is frozen.

February 11. 

At 8 P. M. it is 11° and windy. 

I think it is the coldest day of this winter. The river channel is now suddenly and generally frozen over for the first time. 

P. M. — To Hill. 

The water in the pitcher-plant leaves is frozen, but I see none burst. They are very tightly filled and smooth, apparently stretched. 

The leaves of the round-leaved pyrola, so exposed this winter, look not only dark but as if frozen. I am not sure that they are stiffened however. 

I see that the hemlock leaves also have this frozen or frozen thawed, cadaverous look, dark and slightly imbrowned, especially the most exposed twigs, while some sheltered ones are still a bright green. The same is the case even with the white pines and, as far as I observe, other evergreens. 

There is a change in their leaves with cold weather, corresponding to the reddening and darkening of checkerberry and pyrola leaves. They change, though they do not fall, and are to some extent affected, even as those trees which, like the oaks, retain a part of their leaves during the winter in a withered state; i. e., they have begun to wither or be killed. 

I have often before noticed that the pines, when cold weather came, were of a darker and duller green, somewhat like a frozen apple. In the hemlock, at least, there is a positive tendency to redness. The evergreens, then, though they do not fall the first year, lose their original summer greenness; they are changed and partially killed by the cold, like pyrola and checkerberry and lambkill, and even, in a degree, like oak leaves. Perhaps the pitch pine is the least affected. 

Cut a club of celtis wood. It is hard but, I think, brittle. The celastrus (waxwork) is a soft, spongy, and flexible wood, though of very slow growth. You can easily sink your knife into it. I count twenty-five rings in the heart-wood of one which is not quite an inch in diameter. In the sap there is no evidence of rings at all.

H. D. Thoreau, Journal, February 11, 1858

11° and windy. I think it is the coldest day of this winter. Compare January 23, 1857 ("I may safely say that -5° has been the highest temperature to-day.”) January 9, 1856 ("Probably it has been below zero for the greater part of the day."); February 6, 1855 ("They say it did not rise above -6° to-day.”); February 11, 1855 (“Smith’s thermometer early this morning at -22°; ours at 8 A. M. -10°.”)

The water in the pitcher-plant leaves is frozen.  See November 16, 1852 ("At Holden's Spruce Swamp. The water is frozen in the pitcher-plant leaf.")

The river channel is now suddenly and generally frozen over for the first time. Compare January 20, 1857 ("The river has been frozen everywhere except at the very few swiftest places since about December 18th, and everywhere since about January 1st.”)