]10 ACTION OF LIGHT AND HEAT 



parts become disorganised, from the fracture of the vessels 

 of the cellular system, that the plant itself is said to be 

 frozen. The more water, therefore, plants contain, the 

 more liable they are to injury from frost ; and, according- 

 ly, we find that aqueous plants are those, most easily fro- 

 zen. What parts of a plant should you suppose most 

 liable to be attacked by the frost ? 



Caroline. Those which contain most water : the leaves, 

 where that liquid is conveyed to be evaporated ; and the 

 buds, where, during spring, the sap is brought in such 

 abundance for their nourishment. Besides, the leaves 

 and the buds are most exposed to the air, while the stem 

 and branches are well defended from its inclemency by 

 their warm clothing, the bark. 



Mrs. B. You have judged rightly. If the frost be so 

 inveterate as to attack the stem, it is the alburnum, as 

 being the most moist and tender, which first suffers ; af- 

 terwards the liber, the internal coating of bark. If this 

 freezes, death must ensue, as the vessels which convey 

 the cambium are lodged in this coating. The external 

 layer of bark is the driest of any part of the plant, being 

 constantly subjected to the inclemencies of the season. 

 It is often injured, and in the lapse of time destroyed, and 

 peels off; but it is never frozen. 



Emily. Plants must be more liable to freeze in the 

 spring than in the autumn, at an equal temperature, be- 

 cause they contain more water in the former than in the 

 latter season. 



Mrs. B. True. In autumn the absorption of sap di- 

 minishes, in the spring it increases, both in quantity and 

 celerity of motion, in order to provide for the budding 

 of the plant ; and if in this season a frost should prevail, 

 there is great danger of the plant falling a sacrifice to it. 



Plants of different species vary much in their power 

 of resisting cold. Oaks do not freeze at 56 below the 

 freezing point of Fahrenheit ; beech will support 79, an 

 intensity of cold which congeals mercury. Mr. De Can- 



618. What plants are most easily frozen! 619. Why should the 

 leaves and buds of plants freeze sooner than the steins 1 (320. What 

 parts of the wood will freeze first and which last. 621 . Why will 

 not the bark freeze 1 ? 622. Why are plants more liable to freeze in 

 Spring than in Autumn at an equal temperature 1 ? 623. What are in- 

 stances of different trees freezing at different temperatures'? 



