LECTURES ON BOTANY. 



405 



ticn they usually present the appearance of so 

 many radii of a circle ; they consist of com- 

 pressed plates of cellular tissue, and extend 

 from thn^ pith in the centre of the tnnik or 

 branch quite through the layeis of wood to 

 the bark (see fig. 1). Of their office in the 

 economy of the plant we have no positive 

 knowledge, but they probably act as a medi- 

 um of communication between the outer and 

 inner portions of the trank, and iu the con- 

 veyance of those secretions that are eventu- 

 ally deposited in the old or heart-wood of 

 trees. Dr. Lindley very justly observes that 

 these plates or rays " also serve to bind firm- 

 ly together the whole of the internal and ex- 

 ternal parts of a stem, and they give the pe- 

 culiar character by which the wood of neigh- 

 boring species may be distinguished." " If," 

 he continues, " plants had no medullary rays, 

 their wood vv-ould probably be, in nearly alli- 

 ed species, imdistinguishable, for we are 

 scarcely aware of any appreciable difference 

 in the appearance of woody or vascular tissue ; 

 but the medullary rays (the silver grain of 

 the carpenters) diftering iu abundance, iu size, 

 and in other respects, impress characters up- 

 on the wood which are extremely well mai'k- 

 ed. Thus, in the cultivated cheiry, the plates 

 of the medullary rays are thin, the adhesions 

 of them to the bark are slight, and hence a 

 section of the wood of that plant has a pale, 

 smooth, homogeneous appearance ; but in the 

 wild cherry the medullary plates are much 

 thicker, they adhere to the bark by deep 

 broad spaces, and are arranged with great ir- 

 regularity, so that a section of the wood of 

 that variety has a deeper color, and a twisted, 

 knotted, very uneven appearance. In Quer- 

 cus sessilijlora (the sessile-fruited oak) the 

 medullary rays are thin, and so distant from 

 each other that the plates of wood between 

 them do not readily break laterally into each 

 other, if a wedge is driven into the end of 

 the trunk in the direction of its cleavage : on 

 the contrary, the medullary rays of Quercus 

 pedunculatus (the oak with long-stalked 

 acorns) are hard, and so close together that 

 the wood may be rent longitudinally without 

 difficulty ; hence the wood of the latter is the 

 only kind that is fi.t for application to park 

 paling." 



Observation has proved that each concen- 

 tric layer of which the wood is composed is 

 the produce of a year's growth, insinuated 

 between the next interior layer and the bark. 

 It has likewise enabled us to ascertain the 

 mode in which these successive layers are 

 fonned. In the first place, it is proved posi- 

 tively that they are extended in a downward 

 direction : for example, if during the winter 

 we cut away a portion of the bark of a tree 

 or of a branch, so as to form a contin\iou8 

 wound completely surrounding it — a process 

 sometimes ))erformed in gartlening, and call- 

 ed ringing or girdling, from the figure of 

 the wound — in the succeeding spring, soon 

 after vegetation commences, the upper lip of 

 the cut becomes rounded, in consequence of 

 the formation of woody matter above it, while 



I the lower one remains in the same state in 

 which the knife left it; the part likewise 

 ! above the incision increases sensibly in diam- 

 joeter, the part below does not. The same dif- 

 I ference occurs if a wire or ligature be tightly 

 t bound round the trank or branch : the mat- 

 I ter therefore which causes its increase de- 

 scends. Again, if a gi-owiug branch be cut 

 through immediately below a leaf or bud, 

 that branch never increased iu diameter be- 

 tween the section and the next bud below it; 

 so far, mdeed, from doing so, it becomes dead 

 matter. We know also that the greater the 

 number of leaves or leaf-buds developed on 

 any tree during a season, the more does the 

 diameter of its tiamk become enlarged. These 

 are circumstances all well known, facts tho- 

 roughly ascertained ; and fiom them we ai-e 

 justified in concluding that the successive an- 

 nual deposits of wood are produced by the 

 growth of the buds; and when to them is added 

 the equally substantiated fact, that in the ear- 

 ly spring the incipient or newly fonniug wood 

 may be traced in the form of roots descend- 

 ing from the bases of the already enlarging 

 buds, the phenomenon of the growth or in- 

 crease in the wood of exogenous trees is ex- 

 plained as satisfactorily as the present state of 

 human Science admits. 



Fig. 5. 



WOODY FIBRE DESCK.vniNG FROM BASE OF 

 GROWING HUD. 



The variegated lines and spots, which con- 

 stitiite the chief beauty of wood when man- 

 ufactured, are chietly due to the successive 

 curvatures which these descending root-fibres 

 1 of the buds make in avoiding the previously 

 I developed twigs and branches, and to the in- 

 j eluded basements of those branches, which 

 \ are thus gradually inclosed or siu'rounded by 

 j the fibres of the new wood, forming what 

 j are vulgarly called Jcnois. Hence those ti'ees 

 called dwarf X or ■pollardx, which develop the 

 j greatest number of small branches immedi- 

 1 ately from the trunk, though often less valu- 

 ! able as timber than those which have an erect 



