Anatomy and Histology. 123 



This mechanical difference appears to be due to the nature and 

 extent of the medullary rays and their relation to the wood, together with 

 the relative amount of mechanical tissue in the latter. The very great 

 difference in the size of the parenchyma rays is seen in both transverse 

 and tangential sections, as shown in the figures (plate 28, figs. 1,2; plate 

 33, figs. 3 to 6), in which it is seen that the rays in field plants are very 

 much larger than in irrigated plants. For this reason alone we must con- 

 clude, other things equal, that the former would be much the less rigid. 

 Further, the walls of the medullary-ray cells in irrigated plants become 

 much thickened and lignified (plate 33, fig. 7), while in field plants the 

 cells remain thin-walled indefinitely, with the exception that there occur 

 among them a very few tracheid-like cells (plate 33, fig. 8), with very 

 peculiarly thickened walls. These are so few in number that it is difficult 

 to attach any physiological or mechanical importance to them. The 

 mechanical elements of the wood in the irrigated plants appear more 

 compact to the eye, the lacunas being smaller and the whole mass being 

 made up of smaller and more regularly developed cells. It may here be 

 remarked, also, that the development of medullary stereome is somewhat 

 stronger, but this scarcely contributes a measurable quantity to the total 

 rigidity of a stem more than 1 to 2 mm. in diameter. 



The vessels of irrigated wood are frequently plugged with the so- 

 called " Gummipropfen " at an age of two years or even less. Their 

 appearance is hastened by artificial or by natural wounding, as the dying 

 back of the peduncle. The pith-cells may undergo a considerable amount 

 of sclerosis, without change of shape. The lumen is frequently very much 

 reduced in size, and the walls are traversed by delicate branching canali- 

 culi (plate 29, figs. 5, 6). Sclerosis of pith-cells occurs in Manihot glaziovii 

 near the leaf-bases, that is, at the nodes (Calvert and Boodle, 1887), in the 

 pith of Liriodendron tulipifera (Holm, 1909a), and in that of Cornus flor- 

 ida (Holm, 19096). The sclerosed cells of the last-named are identical in 

 structure with those of the guayule, both as regards the pores and the 

 small size of the lumen. Jodin (1902) also notes a total sclerosis of the 

 pith in Cynoglossum officinale, and partial in Lithospermum fruticosum. 

 The sclerosis of pith-cells under irrigation suggests the value of experimen- 

 tation with other plants in the reverse direction. 



