SECONDARF THICKENING. NORMAL DICOTYLEDONS. 48 1 



The average length, expressed in hundredths of a millimetre, is, for example, accord- 

 ing to Sanio's determination, in 



Tracheides most Fihriform 



similar to vessels. Tracheides, 



Fagus silvatica ...39 75 



Cunonia capensis . . 69 97 



Casuarina torulosa .45 104 



„ equisetifolia 48 75 



Hamamelis virginica .70 80 



Sheperdia canadensis 19 45 



The first category includes those tracheides which also approach the vessels 

 most nearly in the structure of the wall, w^hile those belonging to the second are in 

 this respect also less similar to the vessels, and approach the fibres in every point. 

 If we survey the entire series of the woods investigated, there is on both sides, 

 as well as between the two principal cases just distinguished, a continuous transition 

 between the extremes ^. 



The average thickness of the walls and the nature of the thickening layers ^ 

 generally vary in accordance with the other differences and similarities, if the excep- 

 tional cases of very thick-walled vessels already mentioned be left out of account. 

 The peculiar tubes, containing air, which form the floating apparatus of many 

 Leguminous woods (Herminiera, &c.) must here be mentioned by way of supplement. 

 In order to avoid repetitions, the description of them follows below, in Sect. 150. 



Sect. 143. The sclerenchyma/ous fibres of the wood, shortly termed woody fibres 

 (Fig. 206), are generally distinguished from those elongated tracheides which are 

 more or less similar to them in form, by the structure of their wall. The latter is 

 always destitute of the spirally-thickened innermost layer — although the whole wall 

 may be striated and capable of splitting in a spiral direction, — and its pits, which 

 are invariably slit-shaped, and lie in a left-handed oblique direction, are always 

 present in relatively small numbers, often very sparingly, and differ in details from 

 those of the accompanying vessels. While in many cases they are bordered here 

 also (Quercus, Daphne, Liriodendron, Fraxinus, &c.), they are usually not bordered 

 (Sambucus, Hedera, Clematis Vitalba, Syringa vulgaris, Ligustrum vulgare, Euony- 

 mus latifolius, Celastrus scandens), or so small that the presence of the border is 

 difficult to determine. Both kinds of pits are mentioned by Sanio as occurring in 

 Jatropha Manihot, those with a border being the more numerous. In this plant 

 Sanio found two kinds of pits on the surfaces of junction between the fibres and the 

 medullary rays, namely, small slit-shaped ones on the radial lateral surfaces of the 

 cells of the medullary ray, but large round ones on their horizontal corners, towards 

 which a small pit runs from each adjoining cell of the ray. In the other cases 

 investigated, the pits of the fibres are of approximately equal size, on whatever form 

 of tissue they may border. 



The walls of the fibres are thickened, in a manner which accords generally with 

 the' fundamental rules holding good for all cell-walls ; their usually comparatively 

 thick middle layer ^ is as a rule homogeneous, at least without any conspicuously 



^ Compare Sanio, I.e. pp. 117, 118. '^ See Hofmeister, Pflanzenzelle. p. 196. 



^ Hofmeister, /. c. 



I i 



