Annual 

 Rings. 



Heart- 

 Wood 



Wood 

 Elements 



Bast 

 Elements. 



Stem of 

 Monocoty- 

 ledons. 



is insured between all the lining cells of the stem. The rays 

 first formed extend from the pith to the circumference, while 

 those arising later originate at some distance from the pith. 



In plants the growth of which exhibits periods of activity 

 alternating with periods of rest, there is usually a more or less 

 marked difference between the wood formed at the beginning 

 and end of the period of activity respectively. In the former 

 the vessels are often larger, or more numerous, or, in the 

 absence of vessels, the other elements are usually wider and 

 with thinner walls, and the wood of which they constitute a part 

 contrasts strongly with that which it adjoins formed at the end 

 of the previous season's period of activity, the elements of which 

 are denser. More or less obvious concentric rings thus arise in 

 the wood, which are the so-called annual rings. What is known 

 as heart-wood consists of dead cells, the cavities of which are, as a 

 rule, blocked up by gums, or other substances, and which, being 

 more or less saturated with tannins, is usually of a dark colour. 



The wood of Conifers consists almost entirely of tracheids 

 which have bordered pits chiefly on their radial walls, with some 

 parenchyma, and also sometimes with scattered resin ducts. 



The wood of Dicotyledons consists of vessels, tracheids, 

 parenchyma, and fibres. 



The phloem, or bast, of both Gymnosperms and Dicoty- 

 ledons consists of sieve-tubes, parenchyma, and long, thick- 

 walled, fibres ; the latter, unlike the fibres of the wood, usually 

 have their walls very slightly, or not at all, lignified, and they 

 are therefore tough and flexible. It is owing to this fact that 

 the bast of many species is of commercial value for the manu- 

 facture of ropes and cordage. 



The above elements occur in varying proportions, the wood 

 or bast of a particular species having more of a certain element 

 than that of another species while in some species certain 

 elements may not be represented. 



73. In the young stem of a Mono- 

 cotyledon the primary collateral vascular bundles, instead of 

 being arranged in a circle, are often irregularly scattered 

 throughout the fundamental tissue of the stem. The xylem 

 of each bundle is turned towards the centre of the stem as 

 before, but the bundles are all closed, i.e. no meristematic 

 tissue remains in them to produce secondary growth in 

 thickness. In this case no distinction can be made between 

 the pith and primary medullary rays. In a few Mono- 

 cotyledons, e.g. some Palms, secondary growth in thickness 

 does occur, a cambium ring arising in the fundamental tissue 

 outside the vascular bundles, but this, instead of forming 



