584 LECTURE XXXIII. 



board. A very clear idea of this external work which, together with the internal work, 

 is performed during growth, is obtained by allowing the thick primary roots of active 

 seedlings to grow so that the apices are compelled to penetrate into mercury. This 

 fluid, which is nearly fourteen times as heavy as water, and therefore about fourteen 

 times as heavy as the growing-point itself, of course affords a very powerful resist- 

 ance to the entry of the latter ; nevertheless, the apex of the root penetrates to 

 a depth of 1-3 cm., and would certainly penetrate still deeper if the mercury lying 

 in close contact did not prevent respiration and poison it. 



The external effects in the case of growing tree-trunks with a hard bark are much 

 more powerful, however. I have myself had opportunity of seeing how in still weather 

 a solidly built stone-wall was overthrown simply by the growth in thickness of an old 

 tree. To the more frequently observed phenomena in this direction belongs the slow 

 elevation of huge, powerful, heavy trees by their superficial roots, which are endowed 

 with vigorous growth in thickness. It is a very common occurrence that old Poplars, 

 Pines, and Larches have their upper horizontal roots subsequently protruding above 

 the surface of the earth ; the under side of these roots, which were at first entirely 

 concealed in the soil, bears up against the substratum, which is continually becoming 

 more resistant, and as growth goes on not only are the roots themselves forced to 

 project from the soil, in consequence of this resistance, but also to carry up at 

 the same time the stem, which often weighs many thousands of kilograms. W. S. 

 Clark, who has investigated this phenomenon, reminds us especially of the well- 

 known fact that the delicate seedlings of Beans, Oaks, &c. often push up clods of 

 earth of large size, and states that in England a boundary stone weighing eighty 

 pounds was pushed aside by three large Fungi growing up beneath it ; and the case 

 is known of a Hazel in England which had accidentally grown through the central 

 hole of a mill-stone, completely filled it up, and then being hfted up by the 

 growth in thickness of its roots also carried the mill-stone up with it. In order to 

 study more closely this external work of growth, Clark placed a sort of iron waist-coat 

 (which need not here be described in detail) on a young Gourd fruit, and so arranged 

 matters that a weight of more than 4000 pounds could be brought to bear on the 

 surface of the fruit ; even this did not entirely prevent the growth. Unfortunately 

 the experiment is not described with sufficient exactness to admit of a very clear 

 judgment, but a calculation based on probable data yields the result that a pressure 

 of only eighteen pounds to the square inch was here obtained— little more than 

 the pressure of one atmosphere. 



