482 



SECOND A RF CHANGES. 



apparent finer stratification and striation^; its thickness, however, is very unequal in 

 different species and individuals. The walls are as a rule lignified. A not un- 

 common exception, however, occurs, inasmuch as one of the layers is of a 

 manifestly soft, cartilaginous, gelatinous consistency, and is then excluded 

 from the process of lignification, becoming violet immediately on treatment 

 with preparations of iodine. This gelatinous layer (comp. p. 133) is as a 

 rule the innermost one ; it surrounds the lumen immediately, either as a 

 narrow border (Jatropha INIanihot, Morus alba), or usually as a thick, ap- 

 parently swollen mass, filling the greater part of the lumen. In rare cases 

 a layer enclosed between the lignified ones shows the characteristics in 

 question; these often extend to the whole of the wall lying inside the 

 outermost limiting layer'(' primary membrane '). Lastly, the gelatinous layer 

 may often be distinguished by its refraction, even when it is stained like a 

 lignified membrane by preparations of iodine ^. 



The occurrence of the gelatinous layer is strik- 

 ingly irregular. Sanio found it especially among 

 Leguminosas (Cytisus Laburnum, Sarothamnus, So- 

 phora iaponica, Caragana arborescens, Gleditschia 

 triacanthos), where it is of quite usual occurrence; 

 also in Ulmus suberosa, Celtis australis, Hakea 

 suaveolens, Morus alba, Broussonetia, Ailantus, 

 Fuchsia globosa, Eugenia australis, Castanea, Dios- 

 pyros virginiana, Corylus Avellana, Ostrya vir- 

 ginica, Populus pyramidalis, Betula alba, Alnus glii- 

 tinosa, Enckea media, Eucalyptus cordata, Calycan- 

 thus floridus, Amygdalus communis, Primus Lauro- 

 cerasus, Jatropha Manihot, and Ficus Sycomorus, and 

 he supposes that it occurs much more generally. It 

 is, however, by no means generally characteristic of 

 all the fibres of these woods, as it may be present or 

 absent in different parts even of the same annual 

 ring ; while its occurrence is often rare, and even so 

 isolated (Betula, Alnus) that one may repeatedly 

 investigate a wood without finding it. Nor is its 

 presence or absence connected with any definite 

 special form of structure in other respects, or with 

 the average thickness of the wall. It cannot there- 

 fore be regarded as a characteristic peculiarity of the 

 fibres, the less so, as in particular cases (Hamamelis, 

 Fagus silvatica, Casuarina) it also occurs in elements 

 which, according to their other properties, belong to 

 the class of tracheides which resemble vessels. 



Fig. 206, Fig. 207. 



Fig. 206. — Cytisus laburnum; three-year-old 

 branch during the winter's rest (March). Tangential 

 section {145). abed the zone of secondary growth 

 and cambium bordering on the autumn wood, h of 

 the previous year, and containing a medullarj- ray 

 above. 



FIG. 207. — Cytisus Laburnum. Outlines of a 

 selected short woody fibre, from the youngest annual 

 ring of the same branch as that from which Fig. 206 

 is taken (145). 



What was stated above in the case of the 

 tracheides most closely resembling fibres, applies 

 generally to the fibres themselves as regards form and size. Sanio {I.e. io6) adduces 

 many examples of the occasional bifurcation of their acute ends. On the average 

 their length exceeds that of the neighbouring tracheides the more, the more the latter 



* Compare Sanio I.e. p. 105 



* For details see .Sanio, I.e. p. 10.^. 



