HAZ 



[ 416 



HEA 



Amaryttlds [Amaryllidacece]. Allied to 

 Cooperia.) 



A small bulb, with very narrow leaves and one 

 flowered scape. Offsets ; sandy loam, with a little 

 peat and leaf-mould ; requires the protection of a 

 frame, or to be deeply planted in a dry place in 

 winter. 



H.pusi'lla (dwarf). . Straw. September 

 Buenos Ayres. 182Q. 



HAZEL. Co'rylus avella'na. 



HEADING, or as it is also termed, Cab- 

 baging or Loaviny, is an inaptitude to un- 

 fold the central leaves, characterizing the 

 various members of the Cabbage tribe. 

 They have their centre or bud composed 

 of a larger number of leaves than usual, 

 and these, in some instances, are so com- 

 plexly combined that the plant has not 

 sufficient power to force them open to 

 permit the protrusion of the seed-stem. 

 The closeness of the heading is regulated 

 by the exposure to the light. In a shady 

 situation all the leaves are required to 

 elaborate 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 always 

 have "harder hearts " in summer than in 

 spring or autumn, when the light is less 

 intense. 



HEADING-DOWN, is cutting off entirely, 

 or to a considerable extent, the branches 

 of a tree or shrub a process not rashly 

 to be resorted to, and adopted only to re- 

 duce them when the plant seems declin- 

 ing in vigour, or has attained an undesir- 

 able size. 

 .HEABT'S-EASE. See PANSY. 



HEAT is the prime agent employed by 

 the Almighty Creator to call vegetable 

 life into existence, to develope vegetable 

 form, to effect all vegetable changes, and 

 to ripen all vegetable produce. All these 

 effects are performed most efficiently, in 

 the case of every plant, at some different 

 temperature or degree of heat ; and he 

 who ascertains most correctly those heats 

 has taken a gigantic step towards excel- 

 lence as a gardener. An uncongenial 

 heat is as pernicious to vegetables as to 

 animals. Every plant has a particular 

 temperature, without which its functions 

 cease ; but the majority of them luxuriate 

 most in a climate of which the extreme 

 temperatures do not much exceed 32 

 and 90. No seed will vegetate, no sap 

 will circulate, in a temperature at or below 

 the freezing point of water. No cultiva- 



tion will render plants, natives of the tor- 

 rid 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 in- 

 jured proportionately to the intensity of 

 that reduction. If the reduction of 

 temperature be only slightly below that 

 which is congenial, it merely causes the 

 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 vege- 

 tate in a temperature which is too elevated. 



If blossoms are produced at all, they 

 are unfertile, and the entire aspect of the 

 plant betrays that its secretions are not 

 healthy, and its functions are deadened. 

 Mr. Knight says, " that melons and 

 cucumber plants, if grown in a tempera- 

 ture too low, produce an excess of female 

 blossoms ; but if the temperature 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 this becomes so exces- 

 sive, if it be also promoted by a high 

 temperature, that plants in hothouses, 

 where it has occurred often, dry up as if 

 burned. The justly -lamented Mr. Daniell 

 has well illustrated this by showing, that 

 if the temperature of a hothouse bo 

 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 

 temperature, exhale one hundred and 

 twenty grains in t?ie same space of time. 



Plants, however, like animals, can bear 

 a higher temperature in dry air than they 

 can in air charged with vapour. Animals 

 are scalded in the latter if the tempera- 

 ture 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; but they 

 died in a few minutes if the air was moist. 

 Other plants under similar circumstances 

 would perish probably at a much lower 

 temperature ; and the fact affords a warn- 



