THE STEM OF VASCULAR PLANTS. 169 



The difference in position, also, gives the reason for 

 the diversity which is remarked in the results of experi- 

 ments analogous to those which we have related in 

 speaking of the woody body. If a metallic plate, or 

 wire, be placed between two cortical layers, the foreign 

 body will follow the lot of the bark ; it will gradually be 

 thrown towards the outside, and will come out of the 

 tree as of its own accord. If a nail be fixed in the bark, 

 it will also be thrown towards the outside. If two nails 

 be fixed at the same height, and at a known distance, it 

 will be seen that they gradually have a tendency to sepa- 

 rate, by the thickening of the trunk, and the distension 

 of the fibres of the bark. If a figure, or inscription, be 

 cut on the bark, the letters, without elongating, will 

 become gradually thicker, larger, more separated, more 

 superficial, and finally disappear. Inscriptions upon the 

 bark may, then, although less exactly than those made 

 upon the woody body, cause their date, and that of the 

 tree, to be known. Thus Adanson, having found, in 1 759, 

 two Baobabs, upon the bark of which he observed traces 

 of inscriptions written in the fourteenth or fifteenth cen- 

 tury, remarked that the letters, which were six inches 

 long, only occupied upon the trunk two feet in width ; 

 that is to say, one-eighth of the circumference ; that it 

 was, consequently, probable that they were not written 

 whilst the tree was young. Supposing this case the least 

 favourable of all, and neglecting the rather confused date 

 of the fourteenth century, Adanson estimates that if 

 these trees took two centuries in attaining six feet in 

 diameter, they would require eight, or four times as 

 much, to attain twenty-five feet ; but as the increase of 

 trees, as we have seen above, goes on diminishing in 

 proportion as they become older, one cannot deduce 

 from this observation any exact idea upon the age of 



