BOTANY. 



Vegetable 



Small 

 tu lies. 



Flow of 

 sap. 



Vessels. 



Opinions 



respecting 



them. 



Grew. 



Malpighi. 



Diihamcl. 



Tourue- 

 fort. 



Darwin. 



Knight. 



Central 

 vessels. 



tree, and twist it, and break it partially across, the 

 spiral parts of the trachecc will then be unrolled, and 

 will be very evident. They will resume their natu- 

 ral appearance when the two ends are brought toge- 

 ther, so that they may be allowed to contract. This 

 contraction is extremely well seen in leaves and their 

 foot-stalks, except in those of the Butomus umbellatus, 

 which never contract. They serve also to convey nu- 

 tritious juices to the plant. 



In addition to these larger tubes, there are some 

 which Mirbel calls the small tubes, which are form- 

 ed by the membranes of the cellular net- work. They 

 appear to regulate the density of the wood ; for in 

 proportion to the greater or smaller number that ex- 

 ist in it, will the wood be hard or soft. They com- 

 monly contain colourless and limped juices, and 

 occasionally those that are coloured and thick. Ex- 

 amples of each of these are to be had in the vine 

 and in the pine. 



Such is the result of M. Mirbel's examination of 

 the particular structure of the whole vegetable. 

 Much more has been done in this country ; for the 

 mere existence of tubes and cells has not been ascer. 

 tained without ulterior views. We shall therefore now 

 give a brief sketch of this more particular detail. 



It is sufficiently obvious, that certain fluids exist in 

 plants j and that these fluids are conveyed from one 

 part to another, at particular seasons of the year, 

 seems also to be perfectly ascertained. This fluid is 

 called the sap, and it is observed to flow most freely 

 before the appearance of the leaves : an experiment, 

 illustrative of this fact, is easily made on the vine. 

 From this fluid, the various vegetable matters, such 

 as sugar, gum, acids, and others, are formed : it may 

 therefore be considered analagous to the blood of ani- 

 mals. Our present business is to trace the curious 

 arrangement of the vessels through which it passes, 

 and then to describe the whole of the circulating sys- 

 tem. Much diversity of opinion has appeared on this 

 subject, and it is only very lately that accurate notions 

 have been entertained respecting it. Malpighi and 

 Grew considered the woody fibre which composes 

 the great body of the vegetable kingdom, as the sap 

 vessels; and they employed many plausible argu- 

 ments in support of their opinions : the number, 

 strength, uniformity, and universal distribution even 

 in the most delicate parts of the plants, were all ur- 

 ged ; and although no tubular structure could be 

 discovered, they were considered as the only vessels, 

 because no others could be detected, and it was cer- 

 tain that a circulation was carried on. Duhamel 

 adopted this opinion ; but Tournefort finding the 

 above difficulty press much upon him, resorted to the 

 very singular theory, that the fluids were conveyed 

 through the plant, by a simple capillary attraction. 

 Darwin, in his Phytologiti, seems to have given the 

 first hint of the opinion, which Mr Knight has con- 

 firmed by a series of ingenious experiments, conducted 

 with his wonted accuracy and fidelity. By break- 

 ing gently a twig of a young tree, and by separa- 

 ting the two parts, the vessels will be observed to 

 connect the broken extremities even by the naked 

 eye. These vessels are called, by Willdenow, the 

 adducent vessels, (rasa adducetUia,) and by Mr 

 Knight, the central vessels. They are also the tracheae 



Vegetable 

 Phy.-iolojn 



Alburnoui 

 vessel*. 



Uses. 



or air vessels of Mirbel-and others. They have a spiral 

 form, and have not hitherto been seen on the bark, nor 

 do they appear at the more advanced stage of a branch 

 in which they formerly abounded, for they become 

 woody when the parts grow older. Besides the cen- 

 tral vessels, Mr Knight has described another set that 

 traverse the alburnum, whence they are distinguish- 

 ed by the name of alburnous. Through them the 

 sap also ascends ; for the destruction of a circle of 

 bark does not prevent the formation of buds and 

 leaves ; " but," says Mr Knight, " the alburnous 

 vessels appear to be also capable of an inverted ac- 

 tion, when it becomes necessary to preserve the ex- 

 istence of the plant. The cortical vessels of Mr Cortical- 

 Knight, which can scarcely be considered the same vessels. 

 with the vasa reducentia of Willdenow, (although 

 they are said to perform the same function,) exist in 

 the bark, and serve to reconvey the circulating sap to 

 the root. It is suspected, that there may be two sets 

 of these vessels, one which nourishes the bark, and 

 another that secretes particular fluids in the bark. 

 Lymphatic vessels have also been described ; but we 

 have met with no satisfactory account of them. 



The functions of the vessels of plants have been as va- 

 riously described as the organs themselves. Malpighi 

 supposed them to be air vessels ; Grew declares, that 

 they sometimes contained moisture; and Duhamel 

 suspected that they contained " highly rarefied sap." 

 The experiments already alluded to of Darwin and 

 Knight, have, to a certain degree, determined their 

 uses. The former placed twigs of the common fig 

 tree into a decoction of madder, and on taking them 

 out after some hours immersion, and cutting them 

 across, the coloured fluid was found to have ascended 

 into each branch, and the cut ends of the vessels 

 formed a circle of red dots around the pith, and these 

 vessels again were surrounded by other vessels con- 

 taining the milky juice, so very remarkable in the fig 

 tree. The latter (Mr Knight,) made similar expe- 

 riments with cuttings of the horse chesnut, and of 

 the apple tree, with an infusion in water of very 

 black grapes. The result corresponded with those of 

 Darwin. He, however, pursued the investigation still 

 further, and traced the fluid into the leaves; and du- 

 ring the whole course it did not give the slightest 

 tinge to the bark, nor to the sap between it and the 

 wood. The pith was very slightly, if at all affected. 

 The radicles are probably elongations of these vessels, 

 which aboorb the proper fluids from the earth, and 

 convey it into the body of the root, where it becomes 

 sap by some process which we cannot develope ; 

 it is then conveyed to the stem and leaves, where 

 certain other changes take place, that are to be 

 hereafter noticed. The functions of the alburnous 

 vessels appear to be twofold, according to the views 

 of Knight. At one period, they convey sap to the 

 leaves in common with the central vessels : and du- 

 ring the winter, they serve as reservoirs of the juices 

 of the plant, which, having undergone certain changes 

 in the leaves, are there deposited until the approach 

 of spring, when they contribute to the formation of 

 those new parts which are necessary for the living 

 action of the vegetable. 



The cortical vessels seem to carry the sap back to 

 the roots through the bark, and, in its course, it pos- 



Course oi 

 vessels. 



