Anatomy and Histology. 101 



fig. 4). while the etiolated plant (plate 26, fig. 3) has vessels somewhat 

 fewer, but of more uniform size and notably larger. The mechanical ele- 

 ments of the wood are thicker-walled and somewhat smaller in the field 

 plant (plate 26, fig. 5), and are nearly isodiametric. They are of much 

 the same character in the other two, except that they appear more com- 

 pressed tangentially, especially in the irrigated plant (plate 26, figs. 6, 7). 

 The stereome also presents differences which are still more striking, 

 aside from the relative amounts already spoken of. In the field (plate 

 26, fig. 8) and etiolated (plate 26, fig. 9) plants the cells are closely set 

 together, but are smaller on the whole, and in the field plant have smaller 

 lumina. In the irrigated plant (plate 26, fig. 10) the shape and size vary 

 greatly, the lumina are very small, and the intercellular material is much 

 more extensive. The whole appearance leads to the impression that there 

 is a good deal of distortion during development, so that the fibers are 

 pushed about and disarranged, the tissue becoming less compact. If my 

 view of the origin of the stereome is correct, the explanation of this con- 

 dition may lie in a less complete transformation of the sieve-tissue into 

 stereome. The collapse of the unsclerified cells would cause displace- 

 ment, and the irregularities due to change in position and unequal growth 

 of the stereomatic cells would ensue. The more slowly growing tissues 

 are the more regular and the more compact. The stronger development 

 of mechanical elements in irrigated plants, both in the cortex and stele, 

 appears to be correlated with the larger growth of shoot, while the larger 

 vessels of the etiolated plant indicate the greater proportion of transpir- 

 ing surface (the leaf-surface) to the diameter of the stem. 



LATER SECONDARY STRUCTURE. 



As the hypocotyl approaches a diameter of 3 mm. a total movement 

 outward of the whole vascular system (including the entire wood cylin- 

 der) takes place, a result of the enlargement of the pith and adjacent 

 parenchyma-rays tissue (plate 26, fig. 1 ; plate 28, fig. 3). The inner edges 

 of the hadrome plates or wedges become more or less bent, because their 

 edges are held together unequally by the original solid mass of early sec- 

 ondary hadrome, which splits usually in four places, corresponding appar- 

 ently with the primary parenchyma rays. These, therefore, are at first 

 closed and later opened secondarily, as shown in the figure (plate 25, fig. 

 8), in which the rupture of the xylem cylinder is beginning. In a field 

 plant this expansion of the pith is also accompanied by a considerable 

 tangential growth of the medullary rays. This circumstance, together 

 with the relatively slower rate of growth of wood, brings about the result 

 that in field plants (plate 25, fig. 7) the amount of wood is relatively less 

 than in irrigated plants (plate 25, fig. 8), and the medullary rays are wider. 

 The thickening of the parenchyma rays is shown most strikingly in an 

 etiolated seedling, the consequent rupture of the wood 1 in which is shown 

 in plate 26, fig. 1. 



As to the cortex, the growth has continued in all of its parts in such 

 a manner as to still keep the primary cortical canals included within the 



1 The separation of the young hadrome in succulent roots in this manner is 

 well known. 



