80 DR. V. MAliTIUS 



in regard to the measure, succession, and duration of heat in the 

 tropics. During our winter we miglit approach the actual 

 temperature of the cold months in those countries : but we must 

 not attempt it ; on the contrary, we should endeavour to bring 

 about a compromise between our summer and winter, so as to 

 bring the annual mean temperature below that of tropical coun- 

 tries. The temperature in our large plant-houses ranges probably 

 to the height of ^5° C, or 20° R. In hot summers, provided 

 there is due moisture and atmospheric currents within, the siin 

 will raise that heat to '24° — 30° R. It is obvious that we cannot 

 raise the heat from so dark a source as a stove to equal that 

 emanating from the sun without danger to the plants, because 

 light must be proportionally increased, as must also the hygrosco- 

 pical condition of the air, since by far the greater number of plants 

 experience in their native countries the greatest heat in the wettest 

 season, that is, in that of the rains. We allow the temperature 

 during winter to come down to 12° R. at night, and to 13° and 

 14° during the day, which the plants very rarely experience in the 

 tropics. Mr. Scbott, one of our most successful cultivators, and 

 who has had local experience, considers 16° to 18° R. the most 

 appropriate temperature during day time, lowering at night from 

 14° to 15° R. I agree with him that at Munich a high plant- 

 house will bear 18° R., provided the glass covering is double and 

 the heating apparatus effective enough. 1 need not observe here 

 that this degree of heat, which is two or tliree or even more 

 degrees higher than the usual amount, should be uniform througli- 

 out the building, and that there should be a constant current 

 in the air, so that its different layers may become duly mixed ; 

 otherwise the upper parts of the liouse would be suffocatingly 

 hot and sultry ; I know indeed of a celebrated instance where 

 the plants above require to be frequently shifted or exchanged. 

 Glass covering to the north contributes essentially towards 

 causing such an invisible interchange of the differently heated 

 layers of air, and serves therefore not alone for the admission 

 of light. The ordinary means of ventilation will not produce 

 this salutary effect, however beneficial it may be to Cape or New 

 Holland plants. An hygroscopical increase by means of vapour 

 is here of great importance. A proper system of shading is like- 

 wise indispensable for the due regulation of those two powerful 

 agencies — ligbt and heat. Nevertheless the crowded plants will 

 not always thrive equally well under the most favourable circum- 

 stances ; while some will grow and expand luxuriantly, others will 



