stem, and support the leaves, flowers, or fruits ; as the 

 straw in grasses ; the flower stalks, leaf stalks, &c. 



The Pith is a tolerably firm juicy substance, which is 

 diffused through the inner part of the stalk, to give energy 

 and vigour to the whole ; it is abundant in young plants, 

 diminishes as they grow up, and at length totally disap- 

 pears. 



The Sap is the fluid which nourishes the plant. The 

 warmth of the spring dilates the vessels of plants, produc- 

 ing a kind of vacuum, into which the sap rises ; but, 

 when the cold weather returns, the fibres and vessels con- 

 tract, the sap is forced down into the root; the leaves 

 wither, and are no longer able to perform their offices of 

 transpiration; the secretions stop, the roots cease to ab- 

 sorb sap from the soil, and if the plant be annual, its life 

 then terminates ; if not, it remains in a state of torpor 

 during the winter. The basis of this juice, which the 

 roots suck up from the soil, is water. 



Heat promotes vegetation ; it excites the activity of 

 plants ; it increases the disposition of some of their con- 

 stituent parts for new attraction and combination, to ob- 

 tain such substances as may be requisite and proper for 

 new growth ; it likewise causes them to reject such mat- 

 ters as would be hurtful to them ; it hastens the dissolu- 

 tion or digestion, the formation and secretion of their 

 different products. It enables them to dispose of their 

 superabundant portion of fluids, by promoting perspiration 

 and evaporation. Yet the heat must not be too great, or 

 continued for too long a time, as it occasions a too rapid 

 digestion, and perspiration of their nourishment, and con- 

 sequently an exhaustion. 



Plants have an independent heat of their own. But, if 

 it be difficult to account for the spontaneous production of 

 heat in animal bodies, as all physiologists have found, it 

 must be much more so to account for the generation of 

 vegetable heat. Light, and atmospheric air, at least, arc 

 known to be essential to the vital functions of both. 



In all decomposition, caloric, or heat, is disengaged, and 

 may not the chemical process which takes place within 

 the plant, supply it with the heat which they are acknow- 

 ledged to possess, and which, it is asserted, tempers the 

 cold of the atmosphere ; while the evaporation which takes 

 place through the whole plant, continually moderates the 

 scorching heat of the sun ? 



Dr. Hunter observed, upon this independent heat, that 



by keeping a thermometer placed in a hole made in a sound 

 or healthy tree, it constantly indicated a temperature se- 

 veral degrees above that of the atmosphere, when it was 

 below the fifty-sixth division of Fahrenheit ; whereas the 

 vegetable heat in hotter weather was always several de- 

 grees below that of the atmosphere. The same philoso- 

 pher has likewise observed, that the sap which, out of the 

 tree, would freeze at 32, did not freeze in the tree unless 

 the cold were augmented 15 more. 



But the most remarkable instance of heat in plants 

 upon record, is what is related of the Arum maculatum. 

 Lamarck sa3's, in his Flore Fran^aise, " that when the 

 flowers are in a certain state of perfection, the spadix is 

 so hot as to seem burning, and not at all of the same tem- 

 perature as the surrounding bodies." 



Mr. Senebier noticed that this heat began when the 

 sheath* was about to open, and the spadix\ just peeping 

 forth, and that it was perceptible from three or four o'clock 

 in the afternoon till midnight. 



The Leaves consist of an immense number of fibres or 

 nerves, divided into two sets, one belonging to each sur- 

 face. The surface of the leaf is full of minute pores, 

 through which it imbibes the dew, air, &c. necessary to 

 the growth of the plant, so as to enable it, in some degree, 

 to dispense with supplies from the root ; as we see in 

 plants which live in the water, or swim in that element, 

 which serves them for food ; they have no roots, but receive 

 the fluid at all their pores, and, by decomposition, the 

 hydrogen gas of the water, which constitutes the chief 

 aliment of the plant, is separated, and becomes a principle 

 of the vegetable; while the oxygen gas, the other constituent 

 part of water, is thrown off by the vital forces, escaping by 

 the pores, where the action of light causes its disengage- 

 ment. [See note 1st, upon Gas.] 



Air is also necessary to the growth of a plant. But the 

 air which it requires is not the same appropriated to the 

 use of man. 



Drs. Priestley and Ingenhousz have proved that it is the 



* Sheath, or Spatkea. kind of calyx that opens lengthwise, and 

 puts forth a flower-stalk or spadix, as in the palm arum, &rc. 



It is also applied to the calyx of some flowers which have no spa- 

 dix, as of the narcissus, crocus, iris, <$-c. A membrane investing a 

 stem or branch, as in grasses. 



t Spadix the receptacle proceeding from a spatlic, as in the palm, 

 and some other plants, so called from being produced within a spatha, 

 or sheath. 



