ON PLANTS. 



Mrs. B. It appears by that test, that the trunk of a 

 tree is colder in summer, and warmer in winter, than the 

 air by which it is surrounded ; and the larger the stem, 

 the more this difference is manifest. But the reason of 

 this is very simple : the roots draw their nourishment from 

 a depth of soil, well sheltered from both extremes of heal 

 and of cold : the water they absorb remains throughout 

 the year of a moderate temperature ; and the stem, which 

 serves as a channel to transmit this water to the branches, 

 naturally acquires the temperature of the fluid it conveys* 

 The sap thus tends to cool and refresh the plant during 

 the heat of summer, and to cherish and preserve it from 

 being injured by the severity of winter ; but should this 

 severity be so intense or of such long duration as to pen- 

 etrate into the deep recesses of the soil, whence the sap 

 is drawn, the temperature of the tree will gradually de- 

 scend to that of the atmosphere. 



Caroline. The stem has still another defence from the 

 cold, in the several layers of bark ; which we may, I sup- 

 pose, consider as so many warm coats to preserve the in- 

 ternal temperature of the tree ? 



Mrs. B. Certainly. The bark is a bad conductor of 

 heat, and, like flannel-clothing, serves equally to keep in 

 warmth during winter, and to exclude heat in the summer. 



Notwithstanding all these precautions, which Nature 

 has so wisely taken to preserve the equable temperature 

 of plants, the water within the stem is sometimes frozen ; 

 and as water when converted into ice occupies a larger 

 space than in its fluid state, it bursts the vessels in which 

 it is contained, and injures, if it does not destroy, the 

 texture of the plant. 



Emily. But should this occur after a dry season, so 

 that the cells which contain the water be not filled, there 

 will be room for its expansion in freezing. 



Mrs. B. True ; freezing may then take place with- 

 out considerable injury to the plant : it is only when the 



612. By the thermometer how does the temperature of a tree compare 

 with the air around hi 613. What is the reason for such a result'? 

 614. How will this result be varied provided the severity of winter be 

 of long continuance 1 ? 615. What effect has the bark on the tempera- 

 ture of the tree"? 616. But what will be the effect, if water in the 

 stem of plants be converted into ice 1 ? 617. Under what circumstan- 

 ces will the freezing of plants be attended with the least evil! 

 10 



