136 



THE TISSUES. 



£S0 



contiguous walls. This tissue varies according to trie character of the 

 constituent cells, which are (a) spiral, or (b) annular, or (c) sclariform, 

 or (d) reticulated. 



669. Such cells, with their tapering ends, form vessels with obliquo joints. When 

 porous cells (653) with their truncated ends unite they form right-jointed vessels re- 

 sembling strings of beads, called doited or vascular ducts. These are usually quite 

 large, and characteristic of the woody layers of all exogenous plants. (470.) 



670. The different varieties of trachenchtma are assigned to different re- 

 gions and offices, (a) to the earliest formed part of the wood, the petioles and veins 

 of leaves, petals of flowers, etc. ; (b) to similar parts, but later formed, most abundant 



in ferns and Eqmsetaee;e ; (c) in the woody 

 bundles of the Endogens and in the succu- 

 lent parts of plants in general; (d) most 

 abundant in ferns, club-mosses. 



671. Cienchvma is a system of 

 milk-vessels — vessels secreting the latex 

 or peculiar juice of the plant, white, 

 yellow, red, turbid, containing opium, 

 gamboge, caoutchouc, resin, etc. It 

 occurs in the petioles and veins ; in the 

 parenchyma of roots, in the liber es- 

 pecially ; sometimes simple, generally 

 branched and netted in a complicated 

 manner, as well seen in the poppy, ce- 

 landine, blood-root, gum-elastic tree, 

 etc. « 



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 



673. Intercellular passages filled with air and admitting its free 

 circulation in all directions through the parenchyma. These are neces- 

 sarily very irregular, and they communicate with the external air through 

 the stomata. (§ 678.) 



674. Import of the cell. Thus the cell appears to be the type of 

 every form of tissue, the material of which the vegetable fabric is built, 

 and the laboratory where the work is performed. 



675. Elevation in rank is marked by the increasing complication of the tis- 

 sues. The basis of the structure of all plants is parenchyma. In the lowest tribes 

 no other tissue is ever added, this alone performing all the functions. Higher in the 

 scale, as in mosses, a few central bundles of wood tissue are added, as if to strengthen 

 the stem. Still higher, as in ferns, etc., we begin to find vessels (trachenchyma) of 

 the simpler sort, for the freer circulation of the fluids, together with the strengthening 

 pleurenchyma. Lastly, in the highest plants, Phasnogamia, the true spiral vessels 

 appear, filled with air, cienchyma with secretions, and all the tissues in their appro- 

 priate functions. 



6S1 

 Vessels of Cienchyma ; 5S0, from Dan 

 delion; 5S1, from the Celandine. 



672. Their nature, 



