THE TISSUES. 



129 



390. The pitted cells ( 3 70) constitute a singular variety 

 of wood-fibre, common in Pines, Firs, etc. That mysterious 

 double ring which encircles each pit, is projected, the inner by 

 the pit itself, which is an aperture in the secondary layer, the 

 outer by a lens-shaped intercellular cavity opposite, outside (474). 



391. Trachenchyma is a tissue of vessels or tubes rather 

 than cells. The vessels are extended lengthwise, and composed 

 each of a row of cells joined end to end, and fused into one by 

 the absorption of the contiguous walls. This tissue varies ac- 

 cording to the character of the constituent cells, which arc (a) 

 spiral, or (b) annular, or (c) scalariform, or (d) reticulated. 



392. Such cells, with their tapering ends, form vessels with oblique joints. When 

 porous cells with their truncated ends unite, they form right-jointed vessels resembling 

 strings of beads, called dotted or vascular ducts. These are usually quite large, and char- 

 acteristic of the woody layers of all Exogenous plants. The different varieties of trachen- 

 chyma are assigned to different regions and offices (a) to the earliest formed part of the 

 wood, the petioles and veins of leaves, petals of 

 flowers, etc. ; (&) to similar parts, but later 

 formed, most abundant in Ferns and Equiseta- 

 ceae ; (c) in the woody bundles of the Endogens, 

 and in the succulent parts of plants in general ; 

 (d) most abundant in Ferns, Club-mosses. 



393. Cienchyma is a system 

 jf milk-vessels vessels secreting 

 die latex or peculiar juice of the 

 plant, white, yellow, red, turbid, 

 containing opium, gamboge, caout- 

 chouc, resin, etc. It occurs in the 

 petioles and veins ; in the paren- 

 chyma of roots, in the Liber espe- 

 cially ; sometimes simple, generally 

 branched and netted in a compli- 

 cated manner, as well seen in the Poppy, Celandine, Bloodroot, 

 Gum-elastic tree, etc. 



394. These vessels are probably mere open spaces between 

 the cells at first, subsequently acquiring a lining membrane 

 which never exhibits pores or spiral markings. But there are 

 also true Inter cellular passages filled with air, and admitting its 

 free circulation in all directions through the parenchyma. These 

 are necessarily very irregular, and they communicate with the 

 external air through the stomata ( 397), 



9 



Vessels of Wenrhyma 492, from Dandelion ; 

 493, from the Celandine. 



