M. BIOT ON THE SAP OF PLANTS. 89 



numerous and general characters, which, for the most part, only exist 

 in the imagination of him who established them, and are seldom suit- 

 able to two species of the same genus. The Grands Namaquas call the 

 oricou ghaip, preceding the word with a loud clap. 



M. BIOT ON THE SAP OF PLANTS. 



IN all my experiments, the motion of the sap appears to me to pro- 

 ceed from the eminently hygroscopic quality of the vegetable tissue. 

 The sap received at the roots evaporates by the leaves, whilst between 

 these points the vegetable tissue acts precisely as a cylinder, composed 

 of animal charcoal, covered with an impenetrable envelope, and with 

 its lower part immersed in liquid. The column is thus supplied with 

 all the liquid that it can contain ; the vegetable tissue becomes itself 

 in the state of saturation that suits its mass under the existing tempe- 

 rature. This kind of equilibrium being established, should any cause, 

 a sudden change of temperature for instance, increase the evaporation 

 at the extremity of the branches, these will act by suction ; draw more 

 from the roots, and the equilibrium is still preserved. Should, however, 

 the roots come to furnish more, and the leaves evaporate less, then will 

 ensue turgescence in the vegetable tissue ; and if a hole be made, the 

 sap or liquid will overflow. This is precisely what is observed in the 

 birch tree in spring, when its sap begins to rise, and before its leaves 

 have come forth, or are able to perform their task of evaporation. 



As another trait of resemblance, it may be remarked, that the lateral 

 action of heat on any hygroscopic column, such as we have represented 

 the vegetable tissue to be, would have the effect of rendering it ca- 

 pable of less saturation, and, consequently, would oblige it to throw 

 out a part of the liquid it contains. This is the effect which the sun 

 produces upon the birch, and upon other trees, whose sap runs out at 

 this period. When the leaves come these phenomena cease ; the task 

 of evaporation is performed, and the sap bursts neither from the bark, 

 nor through an orifice, if made. 



Now suppose we replace the impermeable or air-tight envelope by 

 one, on the contrary, capable of absorption from within, and exha- 

 lation from without, the state of things will be changed. The issue of 

 the sap or liquid, by the sides of the envelope, will be more frequent 



VOL. u. NO. n. N 



