i ae YF 5 fy pany Ts z, Te Le ore 4 ' “3 ’ 
Se yates . - oa ee Ss - “A ? ; je ; 
eee Lie Lae oy the Plants 8 Bare 
killed. The cause of death may be partly the coagulation of the al- 
-buminoids, while the decomposition of the cell-wall is perceptible only 
at higher temperatures. Air-dry seeds appear to be able to withstand 
any degree of cold without injury to their power of germination. The 
winter-buds of woody plants, the cells of which contain a great quantity 
of reserve-materials but only a small quantity of water, withstand the 
cold of winter and frequent rapid thawing ; while the succulent leaves at 
the time of their unfolding in the spring succumb to a slight night-frost. | 
Many plants, such as Lichens, Mosses, Fungi of a leathery texture, the 
mistletoe, and some others, appear never to freeze ; while, on the other | 
hand, many flowering plants from a southern climate are killed by rapid 
changes of temperature near the freezing point. Whether the tissue of 
a plant can be killed simply by the solidifying of the water of its cell- 
sap into crystals of ice is uncertain ; while, on the other t.and, it is un- 
questionable that in a great number of plants death is caused only by 
the mode in which the thawing takes place. The same tissue which 
retains its vitality if thawed slowly after freezing, becomes decomposed. 
if thawed rapidly after exposure to the same degree of cold ; so that 
death is evidently in this instance the result not of the freezing but of 
the thawing. Moreover, the lower the temperature at which the freez- 
ing took place, the sooner is the plant killed by the freezing as well as 
by the thawing. A common but not invariable result of the freezing of | 
the cell-sap is that internal cells or layers of tissue are ruptured ; but 
these internal rupturings do not, as was once supposed, always cause 
death ; they have as little to do with the destruction of the life of the 
cells by cold as the splitting of the branches of the trees caused by 
frost, which, when the temperature falls very low, is produced by the 
contraction of the bark and outer layers of wood, the crevices again 
closing when the temperature rises. The phenomena of freezing are, . 
however, at present but imperfectly understood. 
The entire hfe of the plant depends on the action of 
fight on the cells which contain chlorophyll, since this action | 
is essential to the formation of new organic compcunds out 
of the absorbed nutrient substances. But when a certain 
quantity of assimilated substance has been produced under 
the influence of light, a long series of vegetative processes 
may be carried on at its expense without any further direct 
action of light. 
The growth of new organs and the metastasis connected with it 
which takes place in the parts that do not contain chlorophyll, and 
rez 
