218 CULTURE OF ALPINE PLANTS. 



structures. If we only look at the plants discovered when 

 mats are removed from pits which have long been closely 

 covered up during a tedious winter, dropping limb from 

 limb, covered with mouldiness, musty and rotten, instead of 

 the healthy specimens originally placed there, we shall require 

 no argument to show that ventilation in winter is as necessary 

 as in summer, and in cold pits as well as heated buildings. If 

 this is indispensable when plants are in a state of torpor, how 

 much more is it needed in such places as dung-pits or frames, 

 especially where salad, cucumbers, and similar plants are 

 grown. In those cases the object is in part to dry the air, in 

 order that the plants may not absorb more aqueous particles 

 than they can decompose and assimilate. Although plants of 

 this kind will bear a high degree of atmospherical moisture in 

 summer, when the days are long and the sun bright, and when, 

 consequently, all their digestive energies are in full activity, 

 yet they are by no means able to endure the same amount in 

 the short dark days of winter, when, from the want of Hght, 

 their powers of decomposition or digestion are comparatively 

 feeble. 



One of the causes of success in tke Dutch method of winter forcing 

 is, undouhtedly, their avoiding the necessity of winter ventilation, by 

 intercepting the excessive vapour that rises from the soil, and which 

 would otherwise mix with the air. For this purpose they interpose 

 screens of oUed paper between the earth and the air of their houses, and 

 in their pits for vegetables they cover the surface of the ground with 

 the same oUed paper, by which means vapour is effectually intercepted, 

 and the air preserved from excessive moisture. 



It is highly probable, if not yet proved, that the true cause 

 of the difficulty of cultivatiag alpine plants in the lowlands, 

 arises from the impossibiUty of maintaining around them a 

 damp atmosphere associated with low temperature, and the 

 copious evaporation caused by the rapid currents of air to which 

 they are exposed in their native stations. Some plants indeed 

 are able to dispense with these conditions, and live indifferently 

 in elevated regions and on plains ; of which the common 

 Shepherd's Purse {Capsella) and Ca/rdamine hirsuta are 

 Indian examples quoted by Dr. Hooker ; but the majority can 



