1839] 



FARMERS' REGISTER. 



iry 



it undisturbed for several years. On cutting across 

 the bark afterwards, the thread was found To have 

 been pushed outwards by the deposition, between 

 itself and the wood, of new layers, corresponding 

 in number to the years which had elapsed since 

 its introduction. As the bark increases by the 

 addition of a new layer annually, it would seem, 

 that the age of a tree might be determined by 

 counting these layers, as well as those of the 

 wood. This can be done in the case of young 

 trees, whose bark remains entire, but not in the 

 case of an old tree; for as a tree increases in age, 

 the outermost and oldest layers are torn by the 

 Continual distention of the growing stem, and be- 

 coming dead matter, finally fall away. 



The duration of the bark varies in different trees. 

 In the white-oak, (quercus alba,) and many other 

 trees, it breaks into deep longitudinal fissures, as 

 the stem increases in size. The separate portions 

 into which it is thus divided often attain the thick- 

 ness of half an inch or more. On the other hand, 

 a great part of the bark of the sycamore (plmita- 

 nus occidentalis,') falls off every year, in the form 

 of brittle plants, fn the vfh\te-hivch,(^beiula popuU- 

 Jblia,) it separates into their membranous layers, 

 which tear into separate pieces, and gradually 

 curl up, until they curl off. The diHerence observ- 

 able in the manner in which the bark of different 

 trees is rent, arises from a difference in the man- 

 ner in which the woody fibre is disposed in it. In 

 some plants these fibres are disposed perfectly pa- 

 rallel to each other; in others, they cross each 

 other at certain angles. In the celebrated lace- 

 bark tree of Jamaica, they "are numerous, quite 

 white, extremely fine, and when stretched out, 

 open into a kind of delicate net-work resembling 

 lace." 



The bark is obviously intended for the protec- 

 tion of the young wood which is formed beneath 

 it. It IS also the medium through which the juices 

 of a plant, after their elaboration in the leaves, 

 descend, and are transmitted to the wood. Hence, 

 the bark is absolutely essential to the life of exo- 

 genous plants; and there is no more efliectual way 

 of killing them than by removing a section of it 

 all the way around the stem : or girdling them, as 

 it is commonly called. In trees where the fibres lie 

 parallel to each other, or nearly so, the bark may be 

 cut perpendicularly, not only without injuring the 

 tree, but often with decided advantage ; inasmuch 

 as the vessels of the new wood, or alburnum, are 

 in this way enabled to circulate the sap more free- 

 ly. In herbs, and in the young branches of trees 

 where the bark is green and succulent, it performs 

 the same functions as the leaves, and supplies 

 their place, whenever, as in the cactuses, true 

 leaves are wanting. 



Endogenous stems differ very much fi-om exo- 

 genous ones in their structure. The characteris- 

 tic difierences, as I have already mentioned when 

 ■defining these terms, are ihe arrangement of the 

 tissues, and the manner of their growth. Besides 

 this, endogens differ from exogens, in having nei- 

 ther pith, medullary rays, bark, or wood, proper- 

 ly so called, but consisting of a confused mass of 

 woody bundles, imbeded in cellulai- tissue. In the 

 stalk of the corn, (zeamays,) which alibrds a good 

 specimen of a stem constructed on the endogenous 

 plan, we find an external cortical integument, 

 without liber, and bundles of woody matter, soar- 

 fi-anged throughout the cellular tissue, ae to be 



much more numerous and compact at the circum- 

 ference, than towards the centre. In the stem of 

 the garden-asparagus, {asparug^is officinalis,') ihe 

 woody bundles are distribuied unilurmly, through- 

 out the stem, and so soft as scarcely to be recog- 

 nized as woody matter. The same arrangement 

 of the woody bundles exists in the green-brier, 

 (smilax rotudifolia,') the only endoijenous shrub 

 common in Virginia. In the stems of grasses, 

 which have been said to be "the least endoge- 

 nous of all endogenous stems,'" the structure is so 

 modified as not to be at once evident. The pe- 

 culiarity of these stems is, that they are hollow, 

 except at their nodes or joints, which are very 

 compact discs, closing the stem entirely. They 

 are, however, in every instance, at first solid, and 

 become hollow in the course of their growth. In 

 other respects, the stems of grasses present no 

 variation from the typical structure of endogens. 



The slender fibres which appear in endogenous 

 stems, consist of ducts, spiral vessels, and woody 

 fibre, and may be traced as unbroken threads from 

 the base of the leaves throughout the whole lengtli 

 of the stem to the extremities of the roots. Stems 

 of this structure increase in solidity by the annual 

 addition of new woody bundles, within and among 

 those already formed. From this peculiarity in 

 the mode of their growth, they have received the 

 name of endogens, or growers mwardly. In these 

 stems, the older portions of wood are found next 

 the circumference, whilst the newer and softer por- 

 tions lie next the centre. This fiirnishes a good 

 characteristic for distinguishing them from exo- 

 gens, m which the newly formed and soller por- 

 tions are necessarily next the circumference. 



There is no provision made for the growth of 

 the rind of endogens, like that for the bark of 

 exogens; it is, however, capable of distention to 

 a certain extent whilst green ; but when it once 

 hardens, the stem can increase in diameter no 

 longer. The horizontal growth of endogenous 

 plants is therefore limited, except in those rare 

 instances in which the rind remains soA and ca- 

 pable of indefinite distention. Endogenous stems 

 having grown in a horizontal direction lor a cer- 

 tain time with great rapidity, do not afterwards 

 increase in diameter at all. The trunks of palms, 

 and the stems of herbaceous endogens also, suck 

 as that of the asparagus, are almost as large when 

 they first appear, as at any subsequent period of 

 their growth. Ligatures, or the firm embrace of 

 twining plants, by preventing ILirther increase in 

 diameter, soon destroy the life of exogens, whilst 

 they do not affect the growth of endogens at all. 

 The external structure of endogens furnishes no 

 means by which their age can be ascertained. It 

 has been said, that in palms, the external rings, 

 indicating the situation of the leaves, coincide in 

 number vvith the years which the individual has 

 lived. Little dependence, iiowever, can be placed 

 upon this circumstance ; for in many palms these 

 rings have been noticed to disappear alter a term 

 of years ; and besides, it is far from being certain 

 that only a single ring results from eacli year's 

 growth in these plants. 



The life of endogens, as well as their increase 

 in diameter, is limited by the nature of their rind. 

 When the lateral growth of the stem has proceed- 

 ed to a certain extent, the rind hardens, and the 

 stem being in this way prevented from increasing, 

 in diamctiir, can only grow in length ; and as the 



