NCTIUTION OF VEGETABLES. 



47 



issued had sufficient force to raise tlie column of 

 mercury tliirty-two inches and a half above its 

 level, in a few Iiours. Now, the weight of a 

 column of air of the height of the atmosphere 

 is equal to that of a column of mercury twenty- 

 eight inches high, or of a column of water of 

 the heiglit of about thirty-three feet. In this 

 case the force witli wliich the sap rose from the 

 roots into tlie stem, was much greater than the 

 pressure of the atmosphere. 



Many facts and experiments demonstrate the 

 office whicli the leaves perform in the pheno- 

 mena of suction and absorption. Tlius, a 

 branch detached from tlie tree of which it formed 

 part, still absorbs with great power the iluid in 

 which its extremity is immersed. The same 

 action takes place when it is turned upside 

 down, and its summit is immersed in the water, 

 its absorbent power suffering no diminution. In 

 summer, we see that the heat of the sun causes 

 tlie plants which ornament our gardens to shrivel 

 and fade ; but when we examine them at night 

 or in the morning, we find that the dew which 

 the leaves have absorbed has restored their fresh- 

 ness and vigour. 



If a plant be entirely stripped of its leaves, it 

 will soon perish, because the absorption wliich 

 tal^es place by the roots is insufficient to supply 

 all the materials necessary for its nourisliment. 



In many vegetables, and particularly in tlie 

 genus cactus, and other succulent plants, whose 

 roots are very small, and which commonly vege- 

 tate on rocks, or in the shifting sands of deserts, 

 it is evident that the absorption of the nutritious 

 fluid is almost exclusively performed by the 

 leaves and the other parts exposed to the atmos- 

 phere ; for the smallness of the roots, and the 

 extreme dryness of the soil in which tliey grow, 

 would otherwise prevent them from vegetating. 

 From what has been said, it will be seen that 

 the absorbent surface of vegetables, compared 

 with tlieir general volume, is incomparably 

 greater than in animals. 



Course of the Sap. The sap is the colourless 

 and essentially watery fluid which the roots ab- 

 sorb in the eartli, and the leaves in the atmos- 

 phere, for the purpose of supplying nourishment 

 to tlie plant. It contains in solution the true 

 nutritious principles, and deposits them in the 

 interior of the plant, as it passes through its 

 tissue. 



The older phj'siologists were long in doubt 

 respecting the part of the stem through which 

 the ascent of the sap takes place ; some believing 

 it to be the pith, while others considered the 

 bark as the seat of this singular phenomenon. 

 But wlien recourse was had to direct experiments, 

 it was shown that both these opinions were alike 

 erroneous. In fact, the course of the sap is per- 

 formed through the woody layers. The lym- 

 phatic vessels distributed in the wood and al- 



burnum, serve as canals for the transport of this 

 nutritive fluid; and the part nearest the medul- 

 lary tube appears to be the principal seat of tliis 

 ascent. If a branch, or a young plant, be im- 

 mersed in a coloured liquid, the traces of the 

 absorbed fluid may be followed, esj ecially in the 

 vessels near the medullary tube, whereas none 

 of it will be seen either in the pith or in the 

 bark. Coulon accidentally discovered this fact. 

 He had a row of large poplars cut down, when 

 in one of them which had been circularly sawn, 

 and had fallen, but which still held to the stump 

 by its centre, he saw bubbles of liquid and air 

 rising from the inner fibres, and emitting a very 

 distinct sound. He then tried some experiments 

 on the trees which still remained to be cut down. 

 Thus, on having them bored with a large auger, 

 he found that the fragments which were taken 

 from the outer layers of the wood were nearly 

 dry, that they became moister as the auger went 

 deeper, and that when it had arrived at the 

 centre of the stem, the sap began to flow out at 

 the surface. These experiments were laid before 

 the Academy of Sciences, and Desfontaines and 

 Thouin, who repeated them, confirmed their 

 accuracy. This fact then, evidently proves that 

 the ascent of the sap takes place in the woody 

 layers, and especially in those wliich are nearest 

 tlie medullary canal. It has also been shown 

 by experiment, that the progress of the sap is 

 not arrested in trees deprived of their bark, and 

 in which the pith is more or less obstructed ; 

 while in trees from wliieli all the woody layers 

 are removed, it no longer takes place, although, 

 in such as have only a small cylinder of woody 

 layers remaining, the sap may still continue to 

 ascend, as is tlie case in hollow trees, and espe- 

 cially willows, the trunk of which is very fre- 

 quently carious in the interior. 



In tlius passing through the layere of the 

 wood, in its progress upwards, the sap communi- 

 cates with the lateral parts and branches of the 

 stem, cither directly by j unction of their vessels, or 

 by gradually diffusing itself through the minute 

 pores with which the canals that transport it are 

 perforated. The water, which forms the essential 

 basis of the sap, gives off in its progress, and 

 deposits in the vegetable tissue, that matter with 

 which it is impregnated, and which is destined 

 for the nutrition of the plant, and the reparation 

 of its expended fluids. 



When treating of the suction of the roots, we 

 mentioned the experiments of Hales, proving 

 the force with which the ascent of the sap takes 

 place in a stem even of small diameter, it acting 

 with more power upon the mercury tlian a 

 column of air equal to the height of the atmos- 

 phere. Bonnet has also made experiments for 

 the purpose of determining the rapidity with 

 which the sap may rise. Thus, on immersing 

 two stalks of the kidney bean in coloured fluids, 



