336 Knox: STEM OF IBERVILLEA SONORAE 
stems, especially where they widen toward the tuber, they form an 
elaborate ramifying system throughout the periphery and the pith, 
as well as in connection with the supernumerary bundles which 
then develop (FiGUREs 5 and 8). When such stems, perhaps two 
and a half centimeters in diameter, are cut, the contents of these 
_passages ooze out, forming a large viscid transparent drop which 
covers the wound and which immediately hardens, forming a variety 
of wound-gum. When the sieve-tubes retain their content so 
generally, and when the contents are so evidently secretory in 
nature, one seems to be justified in calling them a secretory sys- 
tem of the secondary stem, and might take exception to Solereder 
(28) for saying generally for the entire family ‘‘ Innere Sekretbe- 
halter fehlen.” 
The secondary growth in the sense of the growth of a second 
year is illustrated in FIGURE 9. Its most prominent features are 
the increase in the size of the bundles and in the amount of lep- 
tome and hadrome, the great width of the medullary rays, the 
breaking of the stereome-ring, and the presence of a proniinent 
periderm entirely surrounding the terete stem. One is also 
struck by the absence of any appearance’of distortion due to the 
compression of the tissues, as this is true only in the horny re- 
gions of the leptome. The increase in the size of the bundles 
has been effected by the constant activity of both inner and outer 
cambium. 
The amount of leptome produced is enormous. The outer 
cambium gives rise to all the hadrome elements, vessels, and 
wood-parenchyma, and by its centrifugal divisions also generates 
the outer leptome. It consists of a succession of brick- 
shaped cells which spread ‘across the entire tangential surface of 
the hadrome. Peripheral to this is the mass of the outer lep- 
tome which contains parenchyma and many large conspicuous 
sieve-plates in the sieve-tubes. The inner leptome is similar to 
the outer in character, but the cambium is less regularly dis- 
tributed. It is represented by a group of meristematic cells just 
within the inner tangential surface of the hadrome (FIGURE II Cy): 
These cells are polygonal in outline and are confined to the middle 
of the surface. The lines of cells x and y can be traced back to 
this origin. In many bundles a second series of divisions is local- 
