PHY 



OR, BOTANICAL DICTIONARY. 



PHY 



307 



root sends the food, imbibed by its fibres, to the stem, and 

 what the stem receives from the leaves is constantly employed 

 in the formation of new parts, till either this evolution ceases, 

 from the strength being exhausted, as in annual plants, or 

 till the parts above ground, which can no longer resist the 

 inclemency of the weather, become separated, as in herbs, 

 shrubs, and trees. With the fall of the leaves in ligneous 

 plants, and with the drying of the stem in herbs, all their 

 vegetating powers are exhausted. The great quantity of 

 moisture which the root forwarded to the plant, is consumed: 

 in trees and shrubs, it is employed in the formation of 

 branches, of wood, alburnum, inner bark, leaves, blossoms, 

 and fruit, as well as in the formation of the root; in herbs, 

 in the formation of the parts above ground, the fruit, and 

 the root itself. These fibres, which hitherto conveyed the 

 food, begin to become brittle, and are no longer able to 

 serve this purpose. The sap which circulates in the vessels 

 can no longer produce new shoots above ground, as the 

 temperature is unfavourable. From the moment, then, that 

 the leaves of ligneous plants and the stems of herbs decay, 

 the plant begins to form new radicles in place of the old 

 ones. If at this period, in the latter part of autumn till the 

 middle of January in our climates, a Birch or Walnut is 

 bored, no sap will proceed. The tree indeed has sap, but 

 only as much as it just wants, and as suffices to form new 

 radicles. Hence fruit-trees, which had too much fruit, 

 decay, because their strength by the great waste of sap is 

 too much exhausted. If such a tree or shrub has formed 

 radicles, before the middle of January, those active young- 

 radicles perform their new functions. They imbibe moisture, 

 which they deposit in the cellular texture, and collect in this 

 manner as much sap as the wasting of the powers, which 

 will be necessary in the next summer, requires. If at this 

 time a stem is bored, a great quantity of fluid flows out 

 in those plants which receive a superfluity. But if, at the 

 end of January, or in February, the weather becomes mild, 

 this flow of sap ceases altogether, and trees, if then bored 

 for the first time, give no sap ; a stream of it is observed 

 again when the weather becomes cold. Those who adhere 

 to the theory of ascent and descent of the sap, say, that in 

 warm weather the sap ascended too high, and in cold 

 descended too low. This singular change, however, of its 

 flowing and ceasing to flow, depends on this, that as soon as 

 the weather is fine and mild, the transpiration of plants goes 

 on with greater rapidity; the quantity of the sap, therefore, 

 naturally becomes less ; on the contrary, in cold weather the 

 transpiration is not considerable, and therefore the sap accu- 

 mulates. On this account the roots of herbaceous plants 

 which are collected for medicinal purposes, are more effi- 

 cacious in winter and spring, than in summer, when in full 

 leaf and flower, because, at that time, they have prepared 

 new sap by their young radicles. The circulation of sap in 

 vegetables, cannot be of the same nature as it is found in 

 quadrupeds, birds, fishes, amphibious animals, and insects ; 

 else, we should observe a point from which all the fluids 

 proceeded, and where they again meet together. Were there 

 such a circulation, the Willow could not reproduce new 

 stems from every little branch. The circulation of sap, then, 

 must resemble that which takes place in the polypi, as these 

 also may be dissected into several pieces, from each of which 

 new polypi are again formed. The nature of circulation 

 must be extremely various in the different classes of vege- 

 tables : Impatiens balsamina, being a meadow plant, when- 

 ever it is without water, immediately withers away; but when 

 water is poured upon it, in five minutes after all the leaves 

 and the trunk a^ain stand erect. A tree or shrub will not 



VOL. II. 91. 



recover so quickly. I observed a Cherry tree, the stalk of 

 which was broken immediately under the top, and in which 

 the top was attached to the stem only by a small stripe of 

 bark. It was immediately fastened ; the buds were just 

 opened, but the flowers were still confined ; in about eight 

 days after nothing was observed at the top, it bloomed rather 

 more luxuriantly, but in a short time all of it decayed. 

 I have observed also in the broken-off branches of fruit-trees, 

 that the fruit became ripe ; and also that fruit-trees, the 

 stems of which were frozen, still continued to vegetate, till 

 towards the middle of June; they then decayed. Thus the 

 juice of trees, which is imbibed by the root, appears to be 

 long in reaching the upper part. We must then take for 

 granted, that small coherence by means of the bark in the 

 broken top of the Cherry tree, and in the bough of the fruit- 

 tree, had, as well as the still living wood of the congealed 

 fruit stem, conveyed out of the root a sufficient quantity of 

 sap for some time. Be that as it may, it is certain that the 

 sap of the root is much longer or 'slower in reaching the top 

 of ligneous than of herbaceous plants. A shrub, the roots of 

 which are decayed, or consumed by insects, will for a long 

 time have discoloured leaves, and yet live ; and will even 

 vegetate some time after its root has been destroyed. It is 

 highly probable, that the circulation of vegetables is very 

 complex ; and Cord may not be far wrong when he ascribes 

 a circulatory system to every knot of plants. We may there- 

 fore suppose that the root imbibes the fluids, which (on the 

 admission of heat, and the gases produced by it, are formed 

 by the adducent vessels, particularly those that twine round 

 the air vessels) are communicated to the reducent vessels by 

 means of the cellular texture ; and are again conveyed into 

 the posterior adducent vessels through the same channels, 

 rising by degrees higher and higher till they reach the stalk. 

 Here every knot that envolves a bud, appears to form, with 

 the leaves, a circulatory system, which by means of the pass- 

 ing adducent and reducent vessels, and of the cellular tex- 

 ture, is united to all the other systems, and to the whole 

 plant. According to this principle, I may cut off from the 

 stalk, a small slip, which has only one knot, one bud, and 

 one leaf, and by placing it loose in the ground, render it a 

 plant. As the slip which has its own peculiar circulation is 

 separated from the common circulating system, it will be for 

 a period in a state of inaction, without evolving any new 

 leaf, but in a short time, the vessels, which exist in this par' 

 of the stalk, viz. in the knot, the bud, and the leaf, begin 

 to form a callus below, sending out new parts that become 

 roots ; these young roots soon imbibe nourishment, the bud 

 is evolved, and becomes a young plant, in which again seve- 

 ral systems of circulation are found connected with the whole. 

 The following observation may serve as a proof that every 

 knot has its circulatory system : If, in a young sprout, I cut 

 the knot on which the bud stands, it will not grow, or pro- 

 duce a new plant ; neither will a piece of the stalk grow, in 

 which there is no knot with the bud. As far as our obser- 

 vations extend, the circulation of sap cannot be explained 

 in any other manner. That, according to the differently 

 formed plants, there are more or less anomalous circum- 

 stances or exceptions, is easy to be supposed. But of what 

 nature is the circulation of sap in ligneous plants during win- 

 ter? In all probability, the juices continue to be moved in 

 the same general manner, and to be renovated from the root: 

 their circulation of fluids, however, is slow, because no new 

 parts can be formed in the open air, and because the trans- 

 piration is considerably diminished. The nourishment requi- 

 site for vegetables, is not all derived from the soil in which 

 they are planted, the greatest quantity .being obtained from 

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