17-2 The Influence of the Soil 



Col de Fours, the mountains which form the southern boun- 

 dary of the Allee blanche, and in the department of the lower 

 Alps. 



The northern part of the district is moreover covered with 

 forests of great extent ; they rise from the bottom of the val- 

 lies to a height of 4800 feet, and are chiefly composed of 

 beeches. In the south we find the Abies excelsa, D. C, the 

 larch, the Pinus sylvestris, and Abies picea ; many willows, 

 Alnus incana, the fowlers' service-tree, the hazel, ash, oak, 

 Populus nigra, and, above all, the Acer Pseudo - platanus, 

 which sometimes attains to an enormous size. When this ar- 

 borescent vegetation ceases, it is replaced by another, com- 

 posed of the Pinus pumilio, Alnus viridis, dwarf willows, the 

 Rhododendron ferrugineum, R. hirsutum, Lonicera alpigena, 

 Atragene alpina, and Juniperus nana. An extract from the 

 forest-archives of the country proves that 32 per cent, of the 

 territory is covered with wood ; this report corresponds with 

 what we find to be the case in Bresil, a region which may as 

 yet be considered free from any attempt at cultivation. 



To the forests succeed the alpine prairies, properly so call- 

 ed ; they are composed, for the most part, of the plants which 

 are found among the Swiss Alps. These prairies are very ex- 

 tensive, for they cover about half the surface of the soil. 



Before we enter upon the consideration of the influence of 

 the soil upon the vegetation of the country we are examining, 

 we have to resolve, by way of preamble, some important ques- 

 tions intimately connected with our immediate subject. Let 

 us first endeavour to determine with exactness, the part per- 

 formed by the root in the nourishment of the plant. Duha- 

 mel, Senebier, and Carradori have proved that the root absorbs 

 only by its capillary extremities ; and further, Mohl has made 

 it evident, ('De Palmarum Structural/), that the organization 

 of the root differs altogether from that of the trunk, and that 

 it is the product of a system of gemma, having their point of 

 separation at the base of the stem. At the extremity of the 

 roots the vessels disappear, and are replaced by cellular tis- 

 sue, filled with a fluid, and sometimes with elongated crystals, 

 which I first observed in the second cellular layer of Lemna. 

 At the base of the spongiole we generally find cylindrical 

 hairs, which are frequently recurved ; they multiply in a sin- 

 gular manner under the influence of a humid atmosphere, and 

 in Streptopus amplexifolius they form a thick felt. I think, 

 with Carradori, and contrary to Treviranus, that these hairs 

 perform a prominent part in the absorbing functions of plants. 

 What proves to me the energy of their vitality, is the pheno- 

 menon of cyclosis, which, after M. Meyen, I have observed 



