LIBRIFORM CELLS. 81 



260. The unseptate fibres, the true libriform cells, are only 

 sparingly pitted, except in a few species like Oleander, where 

 they are pitted on both the radial and tangential walls. The 

 pits are generally elongated and oblique, and according to Sauio 

 always running from left to right. 



261. The cell-wall of these fibres is always lignified, and pre- 

 sents three layers ; and in some instances there is also a layer 

 which is plainty gelatinous, e. g., in Betula and Alnus. These 

 gelatinized fibres are not found in all of the annual rings, nor 

 in all parts of even one ring. 



262. Libriform cells are variable in length in different plants ; 

 some of the shortest occurring in Daphne Mezereum, .14 mm., 

 and the longer in Avicennia, 2 mm. In all cases they are the 

 longest elements in the mass of wood. They are generally sim- 

 ple, but occasionally branched cells are met with, as in Tilia and 

 Cladrastis. They are sometimes irregular!}- grouped together, 

 sometimes radiall}- arranged. Species of Magnolia exhibit the 

 latter, Ulmus the former, mode of arrangement. 



263. Septate libriform cells have sometimes been confounded 

 with wood-parenchyma ; but Sanio points out the following 

 distinctive characters : (1) they are always thicker walled ; 

 (2) they have oblique slits, while wood-parenchyma has only 

 roundish pits ; (3) the}' become septate only after the thicken- 

 ing has progressed to some extent, while in wood-parenchyma 

 the divisions begin before the cambium cells 1 from which it ia 

 derived have begun to thicken. 



Septate libriform cells are less common than any other woody 

 element; examples, however, are not rare in Vitis, Hedera, 

 and Rubus. 



264. Vasiform elements. Neither of the two forms already 

 considered namely, typical wood-cells and woody fibres has 

 distinctive spiral markings or true bordered pits (that is, dis- 

 coid markings) ; but another important class of wood-elements, 

 of which mention must next be made, is characterized b}' such 

 thickenings. 



265. To this class of elements it is difficult to give any 

 satisfactory name. They have been collectively termed vascu- 

 lar, but a large part of them are comparatively short and closed, 

 and cannot be property known as ducts or vessels ; the name 

 Tracheal (or Tracheary), more widely employed, is open to 



1 The immediate derivatives from the cambium, which are partly formed 

 woody fibres, have been termed cambium iibres (Sanio : Bot, Zeit,, 1863), 



