404 



MONTHLY JOURNAL OF AGRICULTURE. 



Fig. 2. 



SECTION OF ESnOGENOUS STEM. 



The peculiarity of structure and mode of 

 growth belonging to the exogeii will be best 

 observed and understood by examining the 

 development of the tnuik and branches of 

 one of our ordinary forest or plantation trees. 

 Commence by dividing, across and longitudi- 

 nally, a yearling shoot or newly formed branch 

 of either tree or shrub, and you will find its 

 substance disposed in the following manner : 

 The centre is occupied by a column of cellu- 

 lar tissue, the medulla or ]nlh, around this is 

 a cyhndrical layer of woody matter, technic- 

 ally called the medullary sheath ; and the 

 whole is encompassed by a third portion, con- 

 sisting chiefly of cellular tissue, the bark of 

 the young branch. The stratification of these 

 parts is evident lo the naked eye : their ulti- 

 mate structure is so minute that it can only 

 be traced by the assistance of the microscope. 

 If we pursue our inquiry farther, we shall 

 find that the veins of the leaves are in direct 

 communication with the medullary sheath; 

 in fact, that they are branches or prolonga- 

 tions of the bundles of spiral vessels and 

 woody fibre which compose it, covered with 

 a continuous extension of the cellular tissue 

 of the bark ; while the rudimentary buds in 

 the axils or connecting angles of the leaves 

 and stem, are not only organically connected 

 with the sheath, but contain a projecting por- 

 tion of the medulla likewise. 

 Fie. 3. 



SECTION OF STEM ONE YEAR OLD. 

 (764) 



SECTION OF STEM TWO YEARS OLD. 



Let US now make similar divisions or sec- 

 tions of the branch fi-om which our yearling 

 shoot proceeds — a similar production to itself, 

 only formed during the preceding summer; 

 yet there is a considerable difference in its 

 structure now : the ]:>ith and the medullary 

 sheath remain, but the former has perhaps 

 contracted into half its former compass ; its 

 vesicles are no longer turgid, but dry and 

 empty ; instead of being regularly foimed, 

 they are compressed in various directions; 

 while the latter (the medullary sheath) is 

 scarcely distinguishable from a larger cylinder 

 of woody matter which surrounds and sepa- 

 rates it from the bark with which it was pre- 

 viously in contact; the leaves have fallen, 

 and the points of their union witli it obliter- 

 ated, except so far as they are denoted by the 

 development of the buds which they assisted 

 to nourish into branches, the newly formed 

 shoots we have previously examined. The 

 cause of this alteration, and of the greater di- 

 ameter orlhickness of the last year's branch, 

 remains to be explained : before attempting 

 to do so, let us examine a horizontal section 

 of a branch or trunk (for it is immaterial 

 which we take) in a more advanced stage of 

 growth — say of several years, instead of one 

 or two, as exhibited above in fig. 1. Here is 

 the medulla or pith occupying the centre in a 

 still more compressed state, reduced perhaps 

 to a mere point ; the sheath is proportionally 

 contracted likewise, owing to the pressure of 

 a great accumulation of later-formed substance 

 around it, viz. : the wood, which, whatever 

 may be its thickness, has been gradually in- 

 terposed between the medullary sheath aiid 

 the bark with which it was originally in con- 

 tact. The wood is seen to consist of many 

 concentric layers or cylinders : each layer 

 consists of a more or less compact mass of 

 woody fibre or sap-vessels, and of ducts, 

 which latter are sometimes distributed 

 throughout the layer, but are more frequently 

 disposed toward the inner face of the woody 

 cylinder to which they appertain. There is 

 another remarkable feature in the wood of all 

 exogenous trees — the presence of what are 

 called medullary rays. In a horizontal sec- 



