BOTANY. 



'enable which were followed by a new bark, and by a layer 

 >' "-.y- of new wood; whence Mirbel concludes, that thclat- 



-"~'~' ' tcr is actually derived from the alburnum and ,wood, 

 which first produces the Cambium of Duhamel, or 

 the gelatinous matter already noticed. So that there 

 is a regular circle of operations, the first of which it 

 is not easy to determine. Perhaps the safest conclu- 

 sion that can be deduced will be, that, under certain 

 circumstances, the nature of which is completely con- 

 cealed from us, the wood does form bark. It still 

 remains to be decided whether or not this is the 

 ordinary process of nature: we confess that it does 

 not appear to us to be to. 



ilk. In the midst of all those parts which have been 



just described, is the pith, or medulla, which is a light, 

 spongy, soft substance : in roots it is generally tole- 

 rably firm and juicy, and of a pale green or yellowish 

 colour. Its density and thickness, and other proper- 

 ties, vary considerably in different plants, and even in 

 the same plants at different seasons. It is completely 



surrounded by vessels. Its supposed uses are nearly as 

 numerous as the physiologists that have attempted to 

 discover them. Duhamel considered it of no import- 

 ance, except in so far as it combined the various parts 

 of the plant. Linna-us, as we have already stated, 

 taught that it produced the wood, and, indeed, he 

 considered it the source of all the energy of vege- 

 tation. M-r Knight thinks that the pith supplies mois- 

 ture to the plant when required. Mirbel says that it 

 contains vessels. Dr Smith is inclined to consider it 

 analogous to the nervous system of the animal king- 

 dom. The analogy is, however, very imperfect, for 

 Mr Knight found that a branch might be deprived 

 of the pith without sustaining any injury. 

 ir When the different parts already described are ex- 

 amined, their particular structure is found to be singu- 

 lar and beautiful. We shall first detail the result of 

 M. Mirbel's examination, which may be considered 

 as containing the general features of the vegetable 

 structure; and we shall then give the valuable obser- 

 vations of Mr Knight and others, which afford a more 

 continuous view of the subject. Mirbel describes the 

 vegetable system as composed of cellular substance, 

 and tubes, (Tissu cel/ultiire, and Tissu titbulaire, ) 

 and these he depicts with much minuteness. His 

 own description of the cellular substance is so 

 clear, that we shall, without hesitation, avail our- 

 selves of it. " Ce ne sont point des petites outres 

 ou utricules, comme le disent la plupart des auteurs, 

 ce'ft une membrane qui ss dedouble en quelqite 

 sortt, pour former des vuides contigus les uns aux 

 autres. Dans les parties ou ces cellules n'eprou- 

 vent ancune pression etrangere, elles sont toutes 

 egalement dilatecs, leurs coupes transversales et verti- 

 cales presententdes hexagones semblables aux alveoles, 

 des abeilles ; chaque c6te de ces figures geometriques 

 sont communs a deux cellules, et tout le tissu est 

 d'une regularite admirable, mais lorsqu' une force 

 ctrangere comprime le tissu, les hexagones se defor 

 meat et font place quelquefois a des parallelogrammes 

 plus ou monij alonges. Les parois membraneuses dea 

 cellules sont tresminceset sanscouleur; elles sont trans- 

 parentes comme le verre ; leur organisation est si de- 

 liee, que les microscopes les plus forts ne peuvent la 

 faire apercevoir. Elies sont ordiaairement criblees. 



de pores dont Pouverture n'a certainement pas la Ve^cUble 

 trois-centieme partie d'une ligne, ces pores ont ^JM 

 bordus de petits bourrelets iucgaux et glandulcux, " "~v~" 

 qui interceptent la lumiere et la refractent avec force 

 lorsqu'ils en rc9oivent les rayons. Le tissu ccllulaire 

 est spongieux, elastique sans consistance ; plonge dans 

 eau, il s'altere, ct meme se detruit en pen (L- terns ; 

 il se reduit alors en une espece de mucilage. Les 

 porei etablisent la communication d'une cellule a une 

 autre, etservent a la transfusion des sues, qui est ex- 

 trOmement lente dans le tissu. Je dois meme ob- 

 server qu'il n'est pas conducteur des fluides repandus 

 dans le vegetal, et qu'il ne produit rien parlui meme." 



In the early period of vegetable life, the cellular Tubef- 

 net-work forms a number of lacuna:, which, when 

 more matured, become a regular series of vessels, 

 which have been described with great care by M. 

 Mirbel. Of course, these tubular vessels exist in 

 the root, where they may be perceived by making 

 a transverse section of it. They differ in monocotyle- 

 donous and dicotyledonous plants : in the former 

 they are always found in the middle of woody fibres, 

 or compose the greater part ot them ; while, in the ' 

 latter, they appear to be dispersed at random through- 

 out the wood. Sometimes they form, regular groupes, 

 at other times they are arranged in concentric belts ; 

 and they are particularly abundant around the pith. 

 They pass from the main trunk of the root to all the 

 fibres and appendages, and occasioually form medul- 

 lary rays. 



Mirbel describes four kinds of these large tubes, 

 1. The simple. 2. The porous. 3. The false air 

 vessels. 4. Air vessels.. (Plate LXXI.) 



The first are merely continuous tubes, which com- Simple, 

 monly contain those resinous and oily juices which are 

 known under the name of " proper juices." This kind 

 is most remarkable in green plants, as in. the Eupfior- 

 bice, in Periploca, and in all plants containing very 

 dense fluids. They are most distinct in the bark. 



The coatsof the second are penetrated by small holes, Porous? 

 arranged m regular parallel series around it. The 

 use of this species is not so well understood. It is 

 found chiefly in hard woods, such as the oak. 



The false air vessels are tubes transversely cut by False air 

 parallel fissures, which give it the appearance of ve^elsv 

 having been formed by a series of rings placed above 

 each other, although, in reality, they cannot be se- 

 parated from each otner. These tubes are destined 

 to perform tlie same functions with the porous tubes. 

 They are most numerous in the vine, the substance of 

 which is extremely porous. 



The air vessels, which appears to be an improper Air ve?el. 

 name, resemble the tracheae of insects. The first 

 are formed by the turnings of fibres from right to 

 left. Mirbel declares that he never could discover 

 from a transverse section any opening of a tube ; but 

 that occasionally an ellipse, or the union of two 

 fibres by means of a membrane, were sufficiently dis- 

 tinct. This same author denies the assertions of 

 Malpighi and Reichel, respecting the existence of 

 strictures in the tracheae, and ascribes their error to 

 optical deception. They are chiefly found around 

 the pith, and in the softer parts ot" the vegetable. A 

 very simple experiment will exhibit this organ with 

 great distinctness. Take a young branch of any 



