CHANGES PRODUCED BY NOURISHMENT. 103 



4 



years old ; this he cemented carefully into a glass tube bent 

 in the form of a siphon, filled with mercury ; by the force 

 of the ascending sap alone, the mercury rose at the end of 

 some days to 38 inches. M. Mirbel has confirmed this ex- 

 periment, and added many others of great importance, but 

 which would carry me too far from my subject. 



As the sap circulates in plants by the aid of numerous 

 vessels and cells, which have no rectilinear communica- 

 tion, the force with which the sap ascends may be ex- 

 plained by a principle deduced from the experiments of 

 M. de Montgolfier, who has proved, that, by means of a 

 very small force, liquids may be raised to an almost in- 

 definite height, provided the pressure of the column of 

 liquid be destroyed by numerous interceptions or valves. 



The force with which the sap ascends is proportioned 

 to the health of the plants, and the abundance of its 

 transpiration : a stalk deprived of its leaves will raise 

 less mercury than one retaining them ; and trees having 

 smooth, spongy leaves abounding in exhaling pores, such 

 as the wild quince, the alder, the sycamore, the peach, 

 the cherry, &c., raise it to a much greater height than 

 those of which the leaves are varnished or dry. The 

 beautiful experiments of Hales have verified these results. 



All the water imbibed by the different parts of plants, 

 but especially by the roots, is first employed in mixing 

 the juices, and facilitating their circulation ; it is then 

 decomposed, and a part of it furnishes hydrogen, so abun- 

 dant in the products of vegetation, but the greatest por- 

 tion is evaporated, principally by the leaves, and thus 

 maintains their temperature below that of the atmosphere 

 during the burning heat of summer. Hales observed, 

 that a sun-flower plant transpired by the leaves, in the 

 space of twelve hours, 1 lb. 14 oz. of water. 



The cold which begins to make itself felt in autumn, 

 retards the movement of the sap ; the fluids become 

 thickened, the solids contracted, the leaves cease to in- 

 hale, and the roots no longer absorb nourishment from 

 the soil, and at length the vital functions are suspended. 

 The returning warmth of spring brings renewed life to 

 the organs ; the fluids and the solids receive a greater 

 expansion, circulation is restored, and the sap deposited 

 in the vessels during the summer and earlier part of au- 

 tumn, affords the first nourishment to plants. 



The branches of trees that are lopped off in winter, put 



