BORONIA. 



61 



BOTTOM HEAT. 



and, when planted out, should have a deep 

 rich soil assigned to it. 



Boro'nia (not. ord. Euta'cese). 



A handsome free-flowering greenhouse 

 evergreen shrub : succeeds best in rich 

 fibrous sandy peat. It is propagated from 

 cuttings set in a compost of sand, peat, and 

 charcoal with moderate heat. There are 

 many varieties, all from Australia. 



BossicG'a (nat. ord. Legumino'sffl). 



A neat, elegant, free-flowering genus of 

 plants, greenhouse evergreen shrubs, suc- 

 ceeding best in a compost of rich fibrous 

 peat and silver sand. All the varieties of 

 this plant have been brought from Aus- 

 tralia. 



Bottom. Heat. 



The application of bottom heat, or 

 heat from sources immediately below 

 and under the roots of plants, is 

 nothing more than the adoption and 

 imitation of a natural process. Between 

 the temperature of the air above and that 

 of the soil below there is always a certain 

 relative proportion, and as the temperature 

 of the air rises and falls so will the tem- 

 perature of the soil also increase and 

 decrease, as the case may be, and although 

 the increase or decrease of the temperature 

 of the soil is less rapid and far more 

 gradual than the change in either direc- 

 tion in the heat of the atmosphere, yet the 

 former surely follows the latter, and thus 

 the average proportion is maintained and 

 preserved. Bottom heat, then, which, in 

 other words, is heat applied to the soil in 

 which plants are growing, and consequently 

 to the roots of the plants, is an imitation 

 of this natural process in any structure 

 intended to stimulate and hasten the 

 growth of plants. Let us suppose two 

 cases in which the proper average relation 

 in the temperature of the soil and the 



atmosphere has not been preserved one 

 in whi:h the temperature of the former is 

 too high, and another in which it is too 

 low, in proportion to that of the atmo- 

 sphere. Under the first condition, when 

 the temperature of the soil is too high in 

 proportion to that of the atmosphere, plant 

 food will be absorbed by the roots and 

 transmitted to the leaves at a rate faster 

 than that at which the leaves can assimilate 

 it in a proper manner, and the consequence 

 is an overdue development of shoots and 

 leaves, the suppression, if not the absence 

 of, blossom, and a departure from normal 

 healthy progress. On the other hand, 

 under the second condition, when the 

 temperature of the soil is too low in pro- 

 portion to that of the atmosphere, plant 

 food cannot be absorbed by the roots af 

 the rate that is required by the foliage, 

 stimulated to greater action and con- 

 sequently greater demands for nutriment 

 by the undue warmth of the air, and in 

 the absence of sufficient support, the leaves 

 will flag, droop, and ultimately wither, 

 and the blossoms, or the fruit, if set^ will 

 fall off. 



Bottom Heat, Regulation of. 



The regulation of the temperature of the 

 soil under the application of bottom heat, 

 and its modification at all times of the year 

 in due proportion to the ruling temperature 

 of the air, is thus ably explained by a writer 

 in the " Cottage Gardener's Dictionary," 

 who says : " Every plant obviously will 

 have a particular bottom heat most con- 

 genial to it. Plants growing in open 

 plains will require a higher bottom heat 

 than those growing in the shade of the 

 South American forests, though the tem- 

 perature of the air out of the shade may 

 be the same in each country. That 

 gardener will succeed in exotic plant cul- 

 ture best who, among his other knowledge, 

 has ascertained the relative temperature of 



