WITH REFERENCE TO HORTICULTURE, 37 



hothouse, a depression of temperature, amounting to fifteen or twenty 

 degrees, may easily be produced upon such an evaporating surface. 

 But the greatest mischief will arise from the increased exhalations of 

 the plants so circumstanced, and the consequent exhaustion of the 

 powers of vegetation. Some idea may be formed of the prodigiously 

 increased drain upon the functions of a plant, arising from an increase 

 of dryness in the air, from the following consideration. If we suppose 

 the amount of its perspiration, in a given time, to be 57 grains, the 

 temperature of the air being 75, and the dew-point 70, or the satu- 

 ration of the air being 849, the amount would be increased to 120 

 grains in the same time if the dew-point were to remain stationary 

 and the temperature were to rise to 80 ; or, in other words, if the 

 saturation of the air were to fall to 726. 



Such facts explain why plants in living rooms do not thrive so well as 

 those which are kept in plant structures, the dry air of the living room 

 constantly draining root and top alike of moisture. Hence the fibres 

 in the inside of the pots are alternately moistened and dried, and 

 cooled and heated, and the leaves are deprived of their water by 

 evaporation faster than it can be renewed. 



Excessive dryness of the air likewise interferes with the absorbing 

 and transforming functions of plants, by which new matter is added to 

 them, growth promoted, and all their secretions formed. Plants vary in 

 their power of resisting the evil effects of a dry atmosphere. As a 

 rule, it may be stated that the thinner their leaves the less their ability 

 to withstand it, and vice versd ; hence the great fitness of leathery-leaved 

 plants such as palms, orchids and ficuses, for the decoration of living 

 rooms. 



It is equally or more important to imitate the natural atmosphere of 

 tropical or temperate climes in our hothouses, in regard to moisture 

 than in reference to heat, and with care and attention the one is as 

 easy as the other. The simplest means are generally the best. Tt is 

 specially so in this matter. By sprinkling the leaves and stems of the 

 plants, the floor, sides, and roof of the house, and placing evaporating 

 pans on the heating apparatus, and covering the roof during severe 

 ^v-ather, sufficient moisture may generally be preserved in the air. In 

 vineries these means are often supplemented by the introduction of 

 some sweet fermenting dung, which distils moisture and distributes 

 food at the same time. This, however, would prove fatal in plant 

 houses, and needs skill and caution in using it. An extra dose of 

 ammonia is death to tender plants or young leaves, and there is no 

 difficulty in supplying the air with moisture apart from fermenting ma- 

 terials. To give an idea of the amount required, it may be well to 

 remember that the heat of the glass of a hothouse at night cannot ex- 

 ceed the mean of the external and internal air, and taking these at 80 

 and 40, 20 of dryness are kept up in the interior, or a degree of 

 saturation not exceeding 528. To this, in a clear night, we may add 

 at least 6 for the effects of radiation, to which the glass is more or less 

 exposed, which would reduce the saturation to 434, and this is a 

 degree of drought which must be nearly destructive. No kind of 



