24 



HISTORY OF THE VEGETABLE KINGDOM. 



seen on breaking a brancli of elder, one year old, 

 in which the pith presents the appearance of a 

 green and very moist fleshy body. But, in the 

 progress of vegetation, all these substances, which 

 are in a manner foreign to the proper nature of 

 the pith, disappear, and there remains in the 

 medullary tube nothing but a transparent tissue. 

 In some vegetables, as the stem grows, the 

 meduUary canal becomes in part, and sometimes 

 altogether, empty, tlie whole pith finally disap- 

 pears, and the stem becomes hollow or fistulous. 

 This is observed in many plants of the family 

 of UmbeUifertE. 



Tlie pith communicates with the cellular and 

 herbaceous layer of the bark by means of peculiar 

 prolongations, which it sends through the woody 

 body. To these prolongations, which are dis- 

 posed in a transverse section of the trunk, like 

 rays diverging from the centre to the circumfer- 

 ence, the names of insertions, medullary prolon- 

 gations, or inedullary rays, have been given ; see 

 Plate I. fig. 2. They establish a direct com- 

 munication between the pith and the external 

 cellular tissue of the stem. 



The medullary rays are also to be found, in the 

 greater part, of the thickness of the bark, since 

 they serve to establish a communication between 

 the internal medulla and the external medulla ; 

 but those of the bark have not a direct com- 

 munication with those of the woody layers. 

 Professor Amici has found them to be formed 

 of small porous tubes, transversely placed, con- 

 taining nothing but air, and establishing a com- 

 munication between the internal and extemiil 

 parts of the plant. 



Various opinions have been formed regarding 

 the use of the pith. According to the celebrated 

 Hales, it is the essential organ of vegetation. 

 Being elastic and dilatable, it acts like a spring 

 upon the other parts, which it thus urges on- 

 wards in their developement. Others, again, 

 consider it as a totally inert body. Dutrochet 

 revived the opinion of Hales, and makes the 

 pith perform a very important part in tlie phe- 

 nomena of the growth of vegetables. These 

 opinions will be considered afterwards. 



Such are the various organs which we find on 

 analyzing the stem of dicotyledonous vegetables. 

 All these parts, however, are far ft-om being, in 

 every case, united and visible in the same plant. 

 Sometimes they are so confounded with eacli 

 other, that it is impossible to distinguish and 

 separate them. But, when the most complicated 

 structure of a part is known, it becomes easy 

 to imagine tlie organs which, in certain cases, 

 may happen to be wanting. 



Monocotyledonous stems. In general, the stem 

 of the monocotyledones is more lengthened and 

 more simple than that of the dicotyledonous 

 trees. It is very seldom that it divides into 

 branches, like that which we have just ex- 



amined. The stipe of a monocotyledonous tree, 

 as the palm, when cut across, does not, like the 

 trunk of an oak, an elm, or any other of ' our 

 forest trees, present a regular and symmetrical 

 aspect, ai'isiiig from circular zones of wood, al- 

 burnum, liber, and bark, always disposed in the 

 same order, and a meduUary canal, always occu- 

 pying the central part of the stem. Here all 

 these pai-ts seem united, or rather confounded 

 together. The pith fills up the whole diameter 

 of the stem ; the wood, disposed in longitudinal 

 fasciculi, is scarcely distinguishable in the midst 

 of the medullary substance. The bark does not 

 always exist ; and, when present, it is so little 

 distinct from the other parts of the stem, that 

 its use as an external covering is not apparent. 

 In the dicotyledonous trees, the liardest part iS 

 that which is nearest the centre of the stem, 

 because it is fonned of the oldest woody layers. 

 The reverae is the case in the monocotyledonous 

 trees, the part nearest the circumference being 

 found in them to possess the greatest solidity. 

 In the dicotyledonous trees, the oldest layers 

 are at the centre ; while, in the other class, 

 they occupy the circumference. This will 

 be easily understood, when we shall have ex- 

 plained the peculiar maimer in which the stem 

 of the monocotyledonous trees grows. The 

 fibrous bundles of the stem, which frequently 

 unite together by their lateral parts, so as to 

 fonn a more or less regular network, are, as in 

 the dicotyledones, accompanied by porous vessels, 

 trachesE, and false tracheae, destined to convey 

 the sap, and other nutritious fluids, to all parts 

 of the stem. 



The monocotyledonous trees are therefore 

 distinguished from the dicotyledonous not only 

 by the structure of their seeds, but also 

 by that of their stem. The latter, which is 

 generally simple and cylindrical, does not, like 

 the trunk of the oak and elm, present layers of 

 wood enclosing each other, and disposed regularly 

 around a central canal containing the pith ; but 

 the pith forms the whole thickness of the trunk; 

 and the woody fibres, instead of being collected 

 and brought close to each other, are separated, 

 and have their bundles scattered in the midst of 

 the spongy substance of the pith. 



In Plate I. are represented magnified views 

 of sections of different kinds of wood. Fig. 1 , 

 represents a portion of a thin, transverse slice, 

 taken from a monocotyledonous plant ; the 

 sugar cane. Here there is no appearance of 

 pith, but a uniform cellular mass from the 

 centre to the circumference ; larger openings are 

 seen regularly interspersed among the smaller 

 ones. In the pine tribe, including aU the species 

 of firs, junipers, yew, &c., a vertical section, fig. 

 2, exhibits a regular net work of hexagonal 

 cells ; in the centre is the pith or medulla, a, the 

 lines b b, represent the medullary rays, formed 



