344 
PHYSIOLOGY 
unaided eye) shows how extensive is the branching (fig. 639). Be- 
tween these extremes the bundles run, with lateral connections here 
and there, especially at the nodes, and more or less variation in size 
and branching. 
Tracheal markings. The walls of the tracheae are always peculiarly 
thickened, the thick regions being in the form of rings, or spirals, or a 
network (figs. 640, 641). The thin parts may be more extensive than the 
thick, as in annular and spi- 
ral tracheae (figs. 640, a, s; 
641, s); or they may be 
mere spots in the midst of 
the thick wall, as in pitted 
tracheae (figs. 640, P; 641, 
p, r). The thick and thin 
parts in adjacent tracheae or 
tracheids correspond; and 
thus the movement of water 
laterally, when conditions 
require it, is facilitated. 
In scalariform tracheids the 
parts of the wall not thickened 
are resorbed, and the neighbor- 
ing cavities communicate freely. 
If water in which some 
cinnabar has been rubbed 
up be passed through filter 
paper, to remove all but the very finest particles, and then the fil- 
trate is driven under pressure through a piece of fresh pine wood, the 
pits become choked with cinnabar, showing that water filters through 
them more easily and so in greater quantity than elsewhere. 
Secondary thickening. The primary xylem, i.e. that differentiated 
from the young tissue near the growing points (fig. 642), is adequate to 
supply only the first leaves. As with age the foliage increases, each 
primary xylem strand may undergo secondary thickening; i.e. it has 
added to it similar tissues, originating from a layer of dividing cells 
which adjoins its outer face (fig. 643). In addition, this meristem (cam- 
bium), arising between the primary strands, may originate new strands 
of xylem tissue between the primary ones. These secondary strands 
may then increase in thickness in the same manner as the primary 
mass s P y c 
FIG. 640. Longitudinal section (diagrammatic) 
of a young xylem strand : c, cambium ; y, young 
trachea, un differentiated except as to size ; p, pitted 
trachea; s, s, s, spiral tracheae; a, annular trachea; 
m, pith. After HABERLANDT. 
