SECONDARr THICKENING. NORMAL DICOTYLEDONS. 483 



resemble vessels, comp. Figs. 206 and 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 miUimetre, may 

 serve to demonstrate the relations of length : — 



Tracheides. Fibres. 



Sophora japonica 16 95 



Sarothamnus scoparius ... 17 56 



Ulex europseus 16 103 



Celtis australis 26 87 



Cordia pallida 27 11 1 



Rhamnus cathartica .... 28 52 



.SIsculus Hippocastanum ...26 43 



Tilia parviflora 31 46 



Sahx acutifolia 33 53 



Rhus typhina 32 35 ; 



Rhamnus Frangula 24 44 



Quercus pedunculata .... 49 80 



Prunus Laurocerasus .... 56 126 



Populus pyramidalis • • . • 39 45 



Hakea suaveolens .... s 26 81. 



Eucalyptus cordata 34 60 



Periploca graeca 28 36 



Daphne Mezereum 15 21 



Spiraea chamaedryfolia ....33 35 



Syringa vulgaris 50 51 



Ribes rubrum 49 47 



In the contents 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 Leguminosse, 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 functions 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 stardng-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, mlo fibrous cells, and "^ox^. parenchymatous cells. 



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



I i 2 



