HEA 



[4C5 ] 



HEA 



vegetable form, to effect all vegetable ; curred often, dry up as if burned. The 

 changes, and to ripen all vegetable I justly lamented Mr. Daniell has \velt 

 produce. All these effects are per- illustrated this by showing, that if the 

 formed most efficiently, in the case of j temperature of a hothouse be raised 

 every plant, at some different tempera- j only five degrees, viz. from 75 to 80, 

 ture or degree of heat ; and he who 



ascertains most correctly those heats, j 

 has taken a gigantic step towards ex- I 

 cellence as a gardener. An uncongenial 

 heat is as pernicious to vegetables as 

 to animals. Every plant has a parti- 

 cular temperature without which its 

 functions cease ; but the majority of i 

 them luxuriate most in a climate of | 

 which the extreme temperatures do not ; 

 much exceed 32 and i)0. No seed | 

 will vegetate no sap will circulate at ; 

 a temperature at or below the freezing j 

 point of water. No cultivation will i 

 render plants, natives of the torrid | 

 zone, capable of bearing the rigours of 

 our winters, although their offspring, 

 raised from seed, may be rendered 

 much more hardy than their parents. 

 Others are capable of resisting the 

 greatest known cold to which they can 

 be exposed; yet all have degrees of 

 temperature most congenial to them, 

 and if subjected to lower temperatures, 

 are less or more injured proportionately 

 to the intensity of that reduction. If 

 the reduction of temperature be only 

 slightly below that which is congenial, 

 it only causes the growth of the plant 

 to diminish and its colour to become 

 more pale; this effect being now pro- 

 duced by the plant's torpidity, or want 

 of excitement to perform the requisite 

 elaboration of the sap, as it is by over- 

 excitement when made to vegetate in 

 a temperature which is too elevated. 



whilst the air within it retains the same 

 degree of moisture, a plant that in the 

 lower temperature exhaled fifty-seven 

 grains of moisture, Avould, in the higher 

 temperature, exhale one hundred and 

 twenty grains in the same space of 

 time. 



Plants, however, like animals, can 

 bear a higher temperature in dry air 

 than they can in air charged with va- 

 pour. Animals are scalded in the latter 

 if the temperature is very elevated, and 

 plants die, under similar circumstances, 

 as if boiled. MM. Edwards and Colin 

 found kidney -beans sustained no injury, 

 when the air was dry, at a temperature 

 of 170 j but they died in a few minutes 

 if the air was moist. Other plants, 

 under similar circumstances, would 

 perish probably at a much lower tempe- 

 rature; and the fact affords a warning 

 to the gardener to have the atmosphere 

 in his stoves very dry whenever he 

 wishes to elevate their temperature 

 for the destruction of insects or other 

 purposes. 



Certain plants flourish in hot-water 

 springs, of which the temperature varies 

 between the scalding heats of from 

 l.")0 to 180 of Fahrenheit's thermo- 

 meter ; and others have been found 

 growing freely on the edges of volca- 

 noes, in an atmosphere heated above 

 the boiling point of water. Indeed, it 

 is quite certain that most plants will 

 better bear, for a short time, an elevated 



If blossoms are produced at all, they ' temperature, which, if long continued, 

 are unfertile, and the entire aspect of ; would destroy them, than they can a 

 the plant betrays that its secretions are ; low temperature. Thus a temperature 

 not healthy, and its functions are dead- ! 

 ened. Mr. Knight says, "that melon 

 and cucumber plants, if grown in a 

 temperature too low, produce an excess 

 of female blossoms ; but if the tempe- 

 rature be too high, blossoms of the 

 opposite sex are by far too profuse." 

 The drier the air the greater is the 

 amount of moisture transpired ; and 



much above the freezing point of water, 

 to orchidaceous and other tropical 

 plants, is generally fatal if endured by 

 them for only a few minutes; whereas a 

 considerable elevation above a salutary 

 tomporature is rarely injurious to plants. 

 But this is not universally the case ; for 

 the elegant Primula maryinata is so 

 impatient of heat, that, although just 



this becomes so excessive, if it be also | about to bloom, it never opens a bud if 

 promoted by a high temperature, that } brought into u room in which there is 

 plants in hothouses, where it has oc- \ a fire. 



30 2n 



