SECONDARY THICKENING. NORMAL DICOTVLEDONS. 487 



irregular reticulate rows, between larger, empty, dried-up cells, a condition wTiich recalls 

 the pith of the Roses, &c. In the medullary rays of Atragene alpina annular zones of two 

 kinds alternate from within outwards, the one consisting of a few rows of relatively narrow, 

 closely connected cells, the other of somewhat larger cells connected to form an irregular, 

 coarsely lacunar tissue, but otherwise similar in structure to the narrow celisi This 

 structure owes its origin to the fact, that with each thickening of the ligneous bundles 

 the medullary ray receives an increment of growth, which remains on the whole smaller 

 in the radial direction than that of the ligneous bundles as regards the number and size of 

 its cells. Certain cells or groups of cells either follow the general growth, or are dragged 

 apart in a purely mechanical manner in consequence of it, so as to form the lacuriar zones. 

 In general, though not exactly, each pair of dissimilar zones corresponds to an annual ring. 



In the wood also the cells of the parenchyma are no doubt always accompanied 

 at least by narrow intercellular interstices containing air. In particular cases, to be 

 mentioned below, they surround wide passages containing secretions. 



Sect. 145. Forms of tissue other than those discussed in the preceding para- 

 graphs are in most woods either absent, or, at any rate, of small importance, The 

 most widely distributed are the sacs containing crystals, which, wherever they occur, 

 accompany the parenchyma of the bundles or of the medullary rays ; e. g. Legumi- 

 nosae, as Pterocarpus santalinus, Haematoxylon S Herminiera (pp. 139' and 141), 

 Vitis, &c. 



Laticiferous tubes are abundantly developed in the wood of the Papayaceae, 

 which is chiefly parenchymatous. Their reticulately connected branches are here 

 distributed between the elements of the parenchyma, and are in contact; with the 

 vessels. In other plants containing latex, only those branches which run from the 

 cortex into the pith pass through the secondary wood. No doubt they always exist 

 .earlier than the latter, from the primary differentiation of tissues onwards, and sub- 

 sequently become enclosed, and also of course stretched, by the secondary growth. 

 Where one of these connecting branches borders on woody fibres, the latter are not 

 uncommonly bent inwards so as to follow its course, as is indicated in Fig. 190, p. 438, 

 by the oblique shading. Comp. also Chaps, VI and XII. 



The classification of the elements of the secondary wood given in Sects. 141-145 is 

 based on that which Sanio, in agreement with T. Hartig ", has given in his fundamental 

 works cited above, p. 478. It differs, however, from the latter in some points,' Apart 

 from the medullary rays, Sanio classifies the elements of the ligneous bundles as 



follows : — ■ 



I. Parenchymatous System. 



1. Woody parenchyma. 



2. Fibres representative of the woody parenchyma. 

 II. Libriform System. 



3. Simple undivided bast-like wood-cells or wood-fibres : Libriform 



tissue. • - ) 



4. Septate Libriform tissue. 



III. Tracheal System. 



5. Tracheides, 



6. Vessels. 



His System II includes both our woody fibres and our fibrous cells, the two are placed 

 together on account of their form and the structure of their walls, while no primary 



■ FlUcfciger and Hanbury, Piarmocographia, pp. 176, 188. 

 "Compare especially Botan. Zeitg. 1859, p. 92. 



