532 PHYSIOLOGY. 



happens that more than one ring is formed annually. In our own trees 

 an interruption to the vegetation, such as is caused by an accidental de- 

 foliation during the summer, produces additional annular markings. In 

 the common Beetroot several rings are produced in one season. 



In some tropical trees (Malpighiaceae) the concentric circles are not 

 very clearly marked ; in others they are even separated by a distinct layer 

 of parenchyma. In the Bignoniaceae it is common to find the wood 

 divided into four large portions, separated by wedge-shaped cortical struc- 

 tures, giving in the horizontal section the form of a cross. The old stems 

 of such plants as the Sapindacese &c., above referred to, with isolated 

 bundles outside the central woody cylinder, acquire very anomalous 

 forms with age, since each collection of fibro-vascular bundles is developed 

 annually in its cambium-region, and hence the stem assumes the appear- 

 ance of several stems enclosed in a common bark. In Cycads more than 

 one year is required to complete a woody zone ; thus, in very old stems 

 of Cycads, only a few rings are seen surrounding a voluminous pith. 



Heart-wood and Sap-wood. As woody trunks increase in size, the 

 older parts of the wood frequently go on increasing in density by the for- 

 mation of secondary layers in the cells of the prosenchyma ; thus the old 

 central wood becomes more solid, forming what is called the duramen 

 or heart-wood, which is sometimes deeply coloured by chemical changes 

 or secretion of various substances, as we see in Ebony, Lignum Vitee, 

 &c. The young external layers of wood, in which the ascending current 

 of fluid passes freely, is called the alburnum or sap-wood. The chemical 

 and physical changes which take place as the sap-wood passes into the 

 state of heart- wood have been previously alluded to. 



Origin of the Fibro-vascular Bundles. The fibro-vascular systems of 

 the branches of Dicotyledons originate independently in the bud from 

 the procambium, but soon become blended with those of the parent axis, 

 with which their layers of increase become uninterruptedly continuous. 

 When a branch is broken oft' short, leaving no buds upon it to coutiiine 

 its growth, it becomes surrounded and ultimately entirely enveloped by 

 the succeeding annual layers of wood, and in this way forms a " knot." 

 The numerous small knots of the wood of Pinus sylvestris arise from 

 certain of its branches being broken off while small. 



Pith. The pith or medulla consists of parenchymatous tissue, filled 

 with nutrient matters, stored up for the use of the growing- tissues. It 

 is of most service in young twigs, and becomes more or less inert in after- 

 life, and often disappears as the wood grows. It exists either as a con- 

 tinuous cylinder, or is broken up into disks separated by cavities one 

 from the other, as in the Walnut. The cells of the pith contain starch, 

 or crystals, or simply air. In some cases some of the cells of the pith 

 retain their vitality longer than others, so that there is an admixture of 

 living and dead cells ; and in this way the differences in the pith may 

 even serve to distinguish certain genera one from the other (Gris). Occa- 

 sionally it is more or less completely lignified. 



Medullary Rays. The medullary rays (fig. 579, A) which separate the 

 primary bundles are developed in the cambium-region with the yearly 

 layers of wood, and always extend to the cortical parenchyma ; in the 

 layers of successive years the new elements of the wood separate into 



