234 PLANT ANATOMY, 
wood, logwood, red-wood) in consequence of the thickness of 
the walls of the cells and the absence of dislocating tensions. 
In the bark, however, especially at later periods, in consequence 
of the various dislocations, lacerations, and subsequent divisions 
of the cells, etc., they are seldom distinct (cinnamon). Even 
the bark rays have seldom (for instance, in the willow) a pre- 
cisely radial course (Cinchonas). Nevertheless, the radial rows 
of the elements of the bark may often be followed nearly to the 
epidermis. 
According to the manner in which the distribution of the 
vascular portion and the sieve portion takes place in the vascu- 
lar bundle, the following designations are employed. The bun- 
dle is spoken of as concentric when one of the two parts sur- 
rounds the other in a sheath-like manner, that is, either the 
sieve portion the vascular portion (Rhiz. Filicis, Fig. 135), or 
the vascular portion the sieve portion (Rhiz. Calami, Fig. 151, 
Lthiz. Iridis);' as radial, when the vascular and sieve portions 
are located radially, one behind the other (the typical structure 
of the vascular bundles of roots, Figs. 120, 121); as collateral, 
when both parts run beside each other and are in contact later- 
ally (the typical structure of vascular bundles in the stem and 
leaves of phanerogams). 
In the leaf, the sieve portion always lies in a direction toward 
the lower surface, and the vascular portion always toward the 
upper surface (Fig. 129 g, sb). : 
If, through the activity of the cambium, the formation of 
secondary wood takes place, the primary bundles, which had 
already previously been formed at the tip without the activity of 
the cambium, move more and more toward the interior; and in 
all those cases where they Jie relatively widely separated from each 
other, they project toward the medulla in a bow-like form and 
constitute the medullary-crown or medullary sheath. 
The outermost layer of the fibro-vascular body of roots, which 
Yemains fora long time capable of development, or, in other 
__ words, in a parenchymatous condition, and in which the root- 
lets become formed, is termed the pericambium. 
---——- . Compare Berg's “ Atlas,” Plate xxi., Fig. 51. 
