ON PLANTS. Ill 



dolle found snow-drops in blossom on Mount Saleve be- 

 neath the ice ; and Captain Parry, in his Polar expeditions, 

 discovered many plants, in full leaf and ready to blossom, 

 encased in ice. 



It is remarkable that plants, which are the greatest suf- 

 ferers by extreme cold, are at the same time most liable to 

 injury from intense heat. But this apparent inconsisten- 

 cy admits of an easy solution. We have observed that 

 aqueous plants are easily frozen; they also evaporate abun- 

 dantly ; therefore, when exposed to extreme temperature, 

 whether of heat or cold, they will be either frozen or 

 dried up. 



Plants which secrete a viscous juice do not easily 

 freeze : the tenacity of this thick, sticky fluid prevents, 

 or at least impedes, that arrangement among the particles 

 which is necessary to produce congelation. Freezing, 

 you know, is a species of crystallisation ; and it is requi- 

 site that each particle of liquid should be free to range 

 itself in that order which is essential to the formation of 

 such regular bodies as crystals. 



Caroline. The rising sap then must freeze more easily 

 than the descending sap or cambium, as the latter is 

 thicker and more viscous. 



Mrs. B. Yes ; and there is also another reason wh^r 

 the sap is more liable to free, e than the cambium : the 

 former moves with greater celerity, so that its particles 

 more easily place themselves in the order of crystallisation. 



Emily. Might not recourse be had to the expedient 

 of stripping the plant of its leaves, in order to diminish 

 the velocity of the rising sap, when in danger of freezing? 

 For by depriving a plant of the organs of evaporation you 

 lessen its power of absorption. 



Caroline. But, by preventing the elaboration of the sap 

 in the leaves, you hinder it from acquiring that consist- 

 ence which enables it to resist congelation ; you render 



624. What is stated by Candolle and Parry in illustration of this sub- 

 ject! 625. Why are plants, most liable to suffer from the cold, most 

 liable to suffer from intense heatl 626. Why do plants which secrete 

 a viscous juice not easily freeze! 627. What is freezing said to be, 

 and what is requisite that it may take place! 628. What are the two 

 reasons why the rising sap freezes more readily than cambium! 629. 

 Why does Caroline say the stripping plants of their leaves would not 

 lessen their liability to freeze! 



