208 



HUTCHINSON'S POPULAR BOTANY 



place with the woody bundles in the stems of dicotyledonous trees, such as 

 the Oak, Beech, Elm, etc., though the completion of the circle is accelerated 

 by the formation of new bundles between those already existing. The 

 wood of timber-trees of several years' growth is nothing more than a mass 

 of such bundles closely packed together, and surrounding that part of the 

 ground tissue which is known as the medulla, or pith. Locked in as the 

 pith then appears to be, communication is nevertheless maintained with the 

 bark by means of narrow prolongations of the pith, which, in transverse 

 sections of the stem, have the appearance of lines diverging from the 



centre. These, as having their rise 

 in the medulla, are known as medul- 

 lary rays. They constitute what 

 cabinet-makers call the " silver grain " 

 in wood. 



But how do the woody bundles 

 increase in size ? The question is 

 not easy to answer at least, with- 

 out bringing to our aid a good many 

 technical terms yet we do not des- 

 pair of making the process plain. 

 Here (fig. 260) is represented in 

 transverse section a fibro-vascular 

 bundle from the stem of an herba- 

 ceous plant ; let us examine it. The 

 narrow end, A, is that which, in a 

 complete transverse section of the 

 stem, would be directed towards the 

 centre or pith, while the wide end, 

 5, would of course be nearest the 

 bark (fig. 258). The whole bundle is 

 embedded in ground tissue (gt). Now 

 notice how the vessels are arranged 

 in the bundle. Those adjoining the 

 pith, and represented by darkly lined 

 circles in the midst of other tissue, 

 are annular vessels (a) ; immediately above them, embedded in similar 

 (that is, woody) tissue, are spiral vessels (sp) ; and higher still are pitted 

 vessels of various sizes (pv], surrounded by greatly thickened wood cells. 

 Within the brackets lettered G we have a tissue of delicate growing 

 cells known as the cambium layer ; and above the cambium layer an assort- 

 ment of sieve-tubes, bast-fibres, and pareiichymatous wood-cells, of which 

 the innermost constitute the soft, and the outermost the hard bast. 



The soft, thin-walled, growing cells, or cambium (a name derived from 

 the Latin cambio, I exchange), really divide the bundle into two parts, of 



Fio. 260. FIBRO- VASCULAR BUNDLE. 



Diagram of a transverse section of bundle from an her- 

 baceous plant. (A) Wood or xylem, consisting of (a) 

 annular and (sp) spiral vessels surrounded by cells of the 

 primary wood ; and (pv) pitted vessels in the midst of 

 denser woody cells. (B) Liber or phloem, consisting of 

 (6') hard bast and (b") soft bast, (gt) G-round tissue. 



