SECONDARY THICKENING. NORMAL DICOTVLEDONS. 



483 



resemble vessels, comp. Figs. 266 arid 207 ; in extreme cases it may be approxi- 

 mately equal to that of the tracheides (Syringa), or even somewhat less(Ribes). ■ 



The following mean values, as determined by Sanio in hundredths of a millimetre, may 

 serve to demonstrate the relations of length : — ■ 



Sophora japonica 

 Sarothamnus scoparius 

 Ulex europaeus . . 

 Celtis australis . . 

 Cordia pallida . . . 

 Rhamnus cathartica 

 iEsculus Hippocastanum 

 Tilia parviflora . . 

 Salix acutifolia . . 

 Rhus typhina . . 

 Rhamnus Frangula . 

 Quercus pedunculata 

 Prunus Laurocerasus 

 Populus pyramidalis 

 Hakea suaveolens 

 Eucalyptus cordata . 

 Periploca grsca . . 

 Daphne Mezereum . 

 Spiraea chamaedryfolia 

 Syringa vulgaris . . 

 Ribes rubrum . . 



Tracheides. 



16 . 



17 . 

 16 . 



26 . 



27 . 



28 . 

 26 . 



31 • 



33 . 



32 . 

 24 , 



49 ■ 

 56 . 



39 ■ 



26 . 



34 • 

 28 . 



15 . 



33 • 



50 . 

 49 • 



Fibres. 



■ 95 

 , 56 



103 

 , 87 

 . lit 

 , 52 



43 

 46 



53 

 35 

 44 

 80 

 126 



45 

 Bi 

 60 

 36 

 21 

 35 



■ 51 

 47 



In the conlenls of the woody fibres shrivelled remains of protoplasm and of formed 

 constituents of the contents can only be detected in rare cases where the wall is very 

 thick and the lumen very narrow, as in the tough fibres of the wood of Viscum, and 

 perhaps also of Leguminosae, Quercus, &c. Further attention must, however, be 

 directed to this point, which is difficult to make quite clear, on account of the 

 scantiness of the remaining contents and the thickness of the wall. Even in the 

 cases just mentioned air is certainly present, in addition to the remnants of the 

 contents. In most woody fibres, however, the lumen contains nothing but air and 

 water. It is manifest that they agree in this point with the tracheides, nor does it 

 admit of doubt that in so far as this is the case they take part in the fiinctions of the 

 latter, and thus that we here have a case of the above-mentioned phenomena of 

 incomplete division of labour, The sharp severance of the two organs cannot 

 therefore be carried out v/ithout violence and uncertainty, especially as the characters 

 assigned to them,. and in particular the bordering of the pits, are on the one hand 

 variable in different cases, and on the other are difficult to determine In practice, in 

 the case' of very small pits. It will therefore constantly be necessary to speak of 

 tracheides resembling fibres, and of fibres resembling tracheides. On the other hand, 

 however, many cases of sharp differentiation exist, as in the Leguminosse mentioned, 

 Quercus, and many others ; these cases render the distinction necessary, and by 

 taking these as the starting-point, it can be carried out even in the less clear cases. 



Sect, 144. The cells of the secondary wood may be divided according to their 

 form, YoXa fibrous cells, and %hoi\. pareTichymaloits cells, 



I. The fibrous cells resemble the woody fibres- more or less closely in form. 



