534 SECONDARY CHANGES. 



ring of sclerenchyma (p. 419), and is covered by the very tough epidermis, which is 

 for a time persistent. The vascular bundles take an unequal share in the cambio- 

 genetic growth in thickness, which makes a vigorous start in the next year. The 

 median bundles of the two next higher leaves, occupying two diametrically opposite 

 segments of the circle, and their nexjt neighbours, grow less strongly in thickness 

 than those situated in the two other segments ; the increase is greatest in the three 

 bundles which occupy the middle of each of the latter segments ; it chiefly affects 

 the xylem. During this unequal growth in thickness no perceptible change at first 

 occurs in the form of the cross-section of the whole internode, and even at a later 

 period the change is but trifling. On the other hand, those ligneous bundles, which 

 grow more strongly than the others, press with their inner edges against the pith, the 

 cells of which become compressed in the direction of the corresponding radii of the 

 transverse section, and the general form of the pith is changed in such a manner 

 that its transverse diameter, lying in the direction of the greatest growth in thickness 

 constantly becomes smaller, while the diameter at right angles to the latter remains 

 unchanged. In an internode five or six years old the pith is merely a narrow band, 

 the shorter diameter of which scarcely amounts to jJ^- of the (original) longer one. 

 The cause of these phenomena manifestly lies in the fact that the cortex undergoes 

 too small an extension for the volume of the growing wood, and by its resistance 

 presses the latter against the pith, and compresses it. Allied species of Aristolochia 

 behave in a completely similar way. 



The disorganisation of the pith in the shrubby species of Astragalas, which 

 yield gum-tragacanth, may further be mentioned here, though not actually standing 

 in direct causal connection with the anatomical processes accompanying growth in 

 thickness '- It consists in the conversion of the cellulose-membranes into a mucilage 

 which is capable of swelling up greatly; the change extends from the pith to the 

 medullary rays, and chiefly affects the cells of the middle of the latter, and of the 

 pith, while those which border on the ligneous strands are changed in a less degree, 

 or not at all. In the living plant the mucilage is already present in a highly swollen 

 condition, and is kept in high tension by the pressure of the surrounding resistent 

 tissue =. On injury to these tissues it flows out, and when a plant is left to itself it 

 may spontaneously burst the surrounding tissue, and exude from the cracks in the 

 form of the strings which on drying form the tragacanth of commerce. In many 

 species, e. g. A. rhodosemius, the mucilaginous disorganisation begins early, even 

 immediately below the apex of the stem'; in others it appears to come on at a later 

 period. 



As regards the gradual dying off of the pith in old woody stems, the disappear- 

 ance of the cell-contents, and especially of the store of starch, similar rules hold 

 good to those for the formation of heart-wood. Comp. pp. 403 and 510. 



Leaving out of consideration the displacements obviously occurring in the cases 

 of Aristolochia and Astragalus, no anatomical changes in the wood, directly caused 

 by the growth in thickness, are known. On the influence of the cortical pressure on 

 the formation of autumn-wood, which is indirectly connected with the above, comp. 



> Von Mohl, Botan. Zeitg. 1857, p. 33. 2 Fliickiger and Hanbuiy, PharmacograpMa, p. 153. 



' Graf zu Solms-Laubach, Botan. Zeitg. 1874, p. 69. 



