82 MORPHOLOGY OF THE CELL. 



the objection that while it is a significant term when applied to 

 trachea-like bodies (ducts) it is a misnomer when applied to an 

 elongated cell wholly free from annular or spiral markings. 



266. Tracheal cells are of two chief kinds: (1) those which 

 are closed throughout, at least until a very late stage of devel- 

 opment ; (2) those formed by rows of cells which lose their 

 intervening partitions, and hence are thrown into a long canal, 

 or vessel. The former are known as Tracheids, 1 the latter as 

 Tracheae.; for which terms may be substituted the following, 

 applicable in nearly all cases, Wood-cell and Duct. 



The distinctive markings of tracheids and tracheae arc bordered 

 pits, or discoid markings, and various thickenings of which the 

 spiral ma}' be taken as an example. 



Tracheids and tracheae further agree in the following point : 

 when complete, the protoplasmic mass disappears, leaving gen- 

 erally no trace. The cavity is filled in a few cases with watery 

 fluid, in some with water and air, but in most with air alone. 

 Occasionally other matters may be found in the trachere, for in- 

 stance, latex ; but these are so exceptional as to need no further 

 mention at this point. 



267. Vasiform wood-cells, or tracheids, are elongated and taper- 

 ing cells, more or less lignified, and having peculiar markings, 

 the principal kinds of which, although previously referred to in 

 133, require a more extended treatment here. 



268. Bordered pits, called also areolated dots and discoid mark- 

 ings, are ver\- common, especially in wood of gymnosperms, 

 where they form a characteristic feature both in fossil and 



1 But the term trachcid, as usually understood, is applied to wood-cells with 

 peculiar markings, next to be described. 



The following measurements bySanio show the difference between the length 

 of some tracheids and the libriform cells in the same plant : 



Tracheids. Libriform cells. 



Ehamuus catharticus 28 mm. .52 mm. 



jEsculus Hippocastanum 26 " .43 " 



Daphne Mezereum 15 " .21 " 



Ribesrubrum 49 " .47 " 



Where, however, the tracheids alone are present, they are sometimes much 

 longer ; for instance, in Staphylea pinnata, 1 mm., and in Philadelphus coro- 

 narius, .85 mm. 



According to Sanio, the bordered pits of ducts are the same as those of the 

 tracheids, as regards size, form, and usually as regards frequency. 



Occasionally tracheids are found which are plainly septate. It thus appears 

 that the tracheids form a gradation between true ducts and libriform cells with 

 bordered pits. 



