HE A 



282 



HE A 



instances, are so complex]y combined 

 that the plant has not sufficient power 

 to force them open to permit the pro- 

 trusion of the seed-stem. The close- 

 ness of the heading is regulated by the 

 exposure to the light. In a shady situ- 

 ation all the leaves are required to ela- 

 borate the sap, on account of the defi- 

 cient light rendering each less active; 

 therefore they open as they are formed. 

 In a free exposure a few leaves are able 

 to effect the requisite decomposition ; 

 and hence the reason why cabbages al- 

 ways have " harder hearts'''' in summer 

 than in spring or autumn, when the 

 light is less intense. 



HEADING-DOWN is cutting off en- 

 'tirely or to a considerable extent, the 

 branches of a tree or shrub — a process 

 not rashly to be resorted to, and adapted 

 only to reduce them when the plant 

 seems declining in vigour, or has attain- 

 ed an undesirable size. 



HEART'S-EASE. See Pansy. 



HEAT is the prime agent employed 

 by the Almighty Creator to call vege- 

 table life into existence, to develop 

 vegetable form, to effect all vegetable 

 changes, and to ripen all vegetable 

 produce. All these effects are per- 

 formed most efficiently, in the case of 

 every plant, at some different tempera 



growth of the plant to diminish and its 

 colour to become more pale ; this effect 

 being now produced by the plant's tor- 

 pidity, 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. 



If blossoms are produced at all, they 

 are unfertile, and the entire aspect o 

 the plant betrays that its secretions are 

 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 tempera- 

 ture be too high, blossoms of the oppo- 

 site sex are by far too profuse." The 

 drier the air the greater is the amount 

 of moisture transpired ; and this be- 

 comes so excessive, if it be also pro- 

 moted by a high temperature, that 

 plants in hot-houses, where it has oc- 

 curred often, dry up as if burned. The 

 justly lamented Mr. Daniell has well 

 illustrated this by showing, that if the 

 temperature of a hot-house be raised 

 only five degrees, viz. from 75'' to 80", 

 whilst the air within it retains the same 

 degree of moisture, a plant that in the 

 lower temperature exhaled fifty-seven 

 grains of moisture, would in the higher 



ture or degree of heat; and he who temperature, exhale one hundred and 

 ascertains most correctly those heats, \ twenty grains in the same space of 

 has taken a gigantic step towards ex- time. 



cellence as a gardener. An uncongenial Plants, however, like animals, can 

 heat is as pernicious to vegetables as to bear a higher temperature in dry air 

 animals. Every plant has a particular than they can in air charged with va- 

 temperature without which its functions pour. Animals are scalded in the lat- 

 cease ; but the majority of them luxuri- , ter if the temperature is very elevated, 

 ate most in a climate of which the ^ and plants die, under similar circum- 



extreme temperature does not much 

 exceed 32° and 90°. No seed will 

 vegetate — no sap will circulate — at a 

 temperature at or below the freezing 

 point of water. No cultivation will 

 renderplants, natives of the torrid zone 



stances, as if boiled. MM. Edwards 

 and Colin found kidney-beans sustained 

 no injury, when the air was dry, at a 

 temperature of IG?^; but they died in 

 a few minutes if the air was moist. 

 Other plants under similar circum- 



capable of bearing the rigours of our ' stances, would perish probably at a 



winters, although their offspring, raised 

 from seed, may be rendered much more 

 hardy than their parents. Others are 

 capable of resisting the greatest known 



much lower temperature ; and the fact 

 affords a warning to the gardener to 

 have the atmosphere in his stoves very 

 dry whenever he wishes to elevate their 



cold to which they can be exposed ; yet temperature for the destruction of in- 

 all have degrees of temperature most sects or other purposes, 

 congenial to them, and if subjected to ! Some plants, like some animals, are 

 lower temperatures, are less or more [ able to endure a very high degree oftem- 

 injured proportionately to the intensity I perature. Sir Joseph Banks and others 

 of that reduction. If the reduction of j have breathed for many minutes in an at- 

 temperature be only slightly below that ' mosphere hot enough to cook eggs ; and 

 which is congenial, it only causes the I have myself travelled in Bengal breath- 



