III.] DESTRUCTION OF PLANTS BY FROST 47 



about 3 per cent., while a clay may contain more than 

 10 per cent, of water which the soil particles can hold 

 against the plant, so that it therefore is useless for the 

 support of vegetation. That the roots themselves take 

 water from the soil, and do not merely pass on the 

 suction exerted by the transpiring leaves, may be seen 

 by the manner in which the supply of water to the plant 

 ceases when any such cause as cold stops the functioning 

 of the roots. The vital actions of the root are suspended 

 at or near the freezing point, and water ceases to be 

 taken up ; consequently if the leaves or stem of the 

 plant are subjected to any great drying influence while 

 the roots are thus cold and out of action, the death of 

 the plant by drying-out may result. Indeed, much of 

 the destruction by frost of what are known as tender 

 plants, e.g. tea roses, is due not to drought so much as 

 to cold. Such destruction will always be found most 

 severe if a spell of drying wind comes when the 

 ground is frozen and there is no snow round the plants 

 to maintain a slightly moist atmosphere and prevent 

 the access of the drying wind. The shelter of a little 

 bracken or straw, or of a few spruce boughs, is sufficient 

 to preserve the plants from injury, not that they are 

 thereby maintained at any higher temperature, but 

 because of the protection from wind and evaporation 

 that has been aflbrded. The morning sunshine falling 

 upon a frozen shrub and plant is most destructive, not 

 because the sudden thaw can exert any direct harm, 

 but because the sun starts a considerable transpiration 

 which cannot be met by the roots in the still frozen 

 ground. We may also use the water cultures to 

 demonstrate the fact that the roots, like all other parts 

 of the plant, respire, and therefore must be supplied with 

 oxygen, in place of which they give off carbon dioxide. 

 In a water culture the roots obtain the oxygen they 



