WOODY AND VASCTJLAK TISSUE. 29 



forms the more solid parts of the petioles or stalks of the 

 leaves ; it is seen spread out horizontally in their frame- 

 work, and in consequence of its constant tendency to anasto- 

 mose, its subdivisions or branchlets, which are ramified out 



Fig. 6. 



amidst the green cellular parenchyma, even to the minutest 

 visible fibre, and continued beyond the limits of unassisted 

 vision, run into each other, forming that delicate and beauti- 

 ful network which is visible to the eye in the leaf. It is 

 owing to the vertical development of woody fibre that a 

 piece of wood splits more readily in the direction of the stem 

 (longitudinally) than when broken across (transversely). 



27. "Woody fibre, in the earlier stages of its growth, con- 

 tains fluid, which it carries from the roots, through the stem 

 and branches, into the leaves and extremities of the plant. 

 In the progress of growth, however, its walls become thick- 

 ened by deposits of sclerogen; the caliber of the tubes di- 

 minishes, until, at length, as in the heartwood of trees, their 

 tubular character is wholly obliterated. 



28. The sclerogen thus deposited is generally spread out 

 uniformly over the interior parietes of the tubes, so that bands, 

 lines, and other markings are seldom seen in the walls of the 

 cells of woody fibre. It is necessary, however, to except the 

 fibre of pine woods, which exhibit large saucer-like depressions 

 in the outer walls of the tube, with a dot in the centre, where 



8* 



