STEM OF TILIA. 



155 



readily recognisable by the glistening white walls, strongly thick- 

 ened, especially in the angles. Finally the surface of the stem is 

 occupied by a regularly-developed periderm, the flat cells of which, 

 proportionally to their age (i.e., from within outwards) are more 

 and more brown in colour.- 'A general idea of the structural 

 arrangement can be obtained from the accompanying somewhat 

 diagramatic view of the cross-section of a four-year-old stem of 

 the lime, the description of which is given at the foot of the 

 figure (Fig. 58). 



For a closer study of the wood and bast of Tilia it is again 

 desirable to prepare cross-sections of alcohol material cut from the 

 thickest available stems ; and it is an advantage to lay the pieces 

 selected for work for a few days in a mixture of equal parts 

 alcohol and glycerine. We will examine them in iodine- glycerine. 

 All the elements will be found to be considerably larger than in 

 the thin twigs. The wood of the lime tree consists of vessels (ducts), 

 tracheides, wood- 

 ribres, and wood- 

 parenchyma (Fig. 

 59). The ducts 

 are striking from 

 their considerable 

 breadth (m). They, 

 and also the trach- 

 eides (), so far as 

 they are in contact 

 with other ducts 

 and tracheides, 

 show bilaterally- 

 bordered pits. The 

 wood-fibres (/) are 

 noticeable for their 

 very sparse and ex- 

 ceedingly slender 

 pits. Amongst all 

 these elements the 



wnnrl rarpnphvmfl FlG ' 59. Gross-section through the woo<l of TiUa parvi- 



\\OOCl-paieilcn\ma fii/ . (f (alcohol mat erial). ,. a wide-pitted duct; *, trach- 



cells (p} are readily eides; /, wood-til >re* : p, wood-parenchyma; r. medullary 



A- * i_ j L ra v (> 540). 

 distinguished by 



the protoplasm, and occasionally starch grains, which they 

 contain. In the bast we at once notice the bast-fibres (Fig. 60, /) ; 



