TRANSACTIONS OP THE SECTIONS. 141 



tation in different regions, must assist very materially in removing the veil which 

 now enshrouds the mysterious operations of vegetable life. While the sterility of 

 deserts is to be ascribed, in most cases, to the want of rain, the long droughts to 

 which many extensive plains of the Old and New World are occasionally subjected 

 may be regarded as very unfavourable to arborescent vegetation. It has been long 

 believed that the western prairies of this continent were brought into their present 

 condition by th» labour of former inhabitants, who exterminated the forest, and, in 

 after ages by means of fires, prevented it from regaining possession of the land ; but 

 many facts show that the absence of trees in these localities corresponds to the re- 

 sult which unassisted nature maybe expected to produce. Since my conclusions on 

 this subject were first made known, I have learned that some of them are not new; 

 but my present object is to show that the facts which observation reveals admit of 

 an important generalization, and that these vast plains only exhibit the effects of 

 causes which operate on a greater or less scale in many other parts of the earth. 



As a general rule, mountainous districts and places near the sea are most favoured 

 with frequent supplies of rain ; but the case is different in great plains, especially in 

 those occupying the interior of continents. In some, as in those of the Mississippi 

 Valley, no deficiency is exhibited in the actual amount of rain which falls annually ; 

 but it generally comes in a small number of excessive showers, often separated by 

 very long intervals of dry weather. During these dry periods the elaboration of the 

 sap in trees is carried on in a very imperfect manner; and the woody tissue formed 

 under such unfavourable circumstances must be devoid of proper strength and dura- 

 bility. The tendency to decay which it soon manifests, must be gradually commu- 

 nicated to the whole vegetable structure, and thus a dry season inflicts a very 

 serious and permanent damage on the forest ; but though it may exterminate the her- 

 baceous plants, the loss will be speedily repaired by the copious rains of the 

 succeeding year. 



The effects of these circumstances on vegetation may be traced in many regions. 

 Trees of the same kind attain the greatest age and afford the most durable timber,' 

 in places where rains are supplied in the greatest frequency, — as on islands, on the 

 sea-coasts of continents, or on mountainous districts. The most numerous and the 

 most extraordinary cases of arborescent longevity, are to be found in the islands of 

 Sicily and Teneriffe, in the mountains near the Syrian coast, in the mountainous 

 territory of California, in the forests of Guiana, and in the British Isles. It is different 

 on plains, especially in those places which are remote from the sea. The forests of 

 European Russia, though very extensive, rarely furnish very durable timber; and the 

 Russian ships are characterized for their great liability to decay. It is also well 

 known that the timber of the Mississippi Valley is far less durable than that of 

 the states bordering on the Atlantic; and the increasing number of hollow trees 

 which we meet on retiring from the sea-coast, may be regarded as indicative of the 

 feeble health and the declining condition of the western forests. 



It could not be expected that even the more gigantic vegetable forms could long 

 withstand influences so detrimental to their health and vitality. Accordingly in ex- 

 tensive continental plains, the forest, by a constant degeneracy of its members, must 

 be often rendered incapable of spreading its dominion, or of contending successfully 

 with the grass for the possession of the soil. The fertility of these plains, by pro- 

 moting a more rapid growth of the wood, increases its tendency to decay ; and ac- 

 cordingly trees are generally absent from the more fertile parts of the prairies, while 

 they are to be found in localities where the land is too poor to give an undue 

 luxuriance to vegetation. They are also found growing vigorouslv along the banks 

 of rivers, where the soil has the greatest fertility; but here the watery vapour which 

 constantly rises into the air diffuses copious dews around, and compensates, to some 

 extent, for the deficiency of rains. 



That the evaporation of the water which falls on the leaves of plants is concerned 

 in promoting their vegetative functions, has been noticed by Boussingault ; but many 

 facts show that its influence is chiefly felt in the formation of woody fibre. The 

 lignifying process, however, depends as much on the extent of the foliage as on the 

 frequency of rains ; and according to Loudon, the pruning of forest trees has been 

 always found very detrimental to the durability of their wood. But experience has 

 Jong shown the necessity of pruning fruit trees ; and it appears that as the lig- 



