UPON PLANT-HOUSES. 79 



temperature of the air in the shade was at the same times 98° F. 

 (= 29° 33 R.) and 92° F. (= 26° 27 R.). At five p. m., and 

 four inches below the surface, the ground which had been much 

 shaded, had still a teaiperature of ]02° F. He points out that 

 these observations prove that in the hot months at the Cape, roots 

 and bulbs, which do not derive their support from any great depth, 

 endure commonly a temperature which could only be imitated in 

 our hot-houses by holding red-hot iron plates over the earth ; for 

 it is to be remembered that bottom heat imparted from below 

 would by no means disti'ibute such a degree of temperature." So 

 far the experienced Lindley. 



I need not observe that the bottom-heat of our glasshouses, 

 that is where bark-beds are not used, is not very high ; so that if 

 a person were to sleep during a night on the floor, he would not 

 escape unhurt, as travellers do in hot zones. In places where the 

 ground is heated by subterraneous fires and solfatores tropical and 

 subtropical plants often thrive well ; they are thrown back at the 

 beginning of the cold season, but in the warm nest below the sur- 

 face that part is sufficiently supported to shoot forth again in 

 spring. These are facts which ought to guide us ; and as a proof 

 of this I may instance the splendid effect of the ground heat in 

 the grand palm-house at St. Petersburgh. 



The consequence of a diminished degree of heat in the root 

 and stem system of tropical forest trees is condensation of the 

 wood and tardiness in flowering resulting therefrom ; for although 

 the monocotyledons among them are unremittingly forming fresli 

 inflorescences, yet these remain for the most part concealed and 

 in an undisclosed state within the base of the leaves to be brought 

 forth only during an especially favourable summer. The Agave 

 Americana blossoms proverbially only once in a hundred years; 

 although in the southern parts of Europe this period is much 

 shortened, and within the tropics still more. All this demonstrates 

 that a certain and successive degree of heat produces regularity in 

 the flowering periods, especially among monocotyledons ; and 

 therefore it is that the experienced cultivator, in order to attain 

 this object, transports the hardy blossoms from a high to a low glass- 

 house. But dicotyledonous trees which have been long kept 

 under a low temperature have their wood often so much condensed 

 that neither the pruning-knife nor anything else will make them 

 produce even a rudimentary inflorescence. 



In general, we can more readily command the temperature of 

 the air than the ground in our plant-houses ; but still we fall short 



