PLANT TISSUES 147 
the branch supply consists of two bundles, these soon unite on 
their way into the branch to form a cylindrical stele which 
appears circular as viewed in cross section. In instances where 
only one trace or bundle strand courses into the branch, this 
appears in cross section as a horseshoe or crescentic structure 
with an opening. The opening is closed as the trace passes 
into the branch so that in the branch a complete cylindrical 
stele is formed. 
LEAF AND Brancu Gaps.—These are breaks in the continuity 
of the vascular cylinder of stems around and above the places 
where the leaf or branch traces leave the cylinder. The break 
or opening is designated as a leaf gap or branch gap according 
as to whether a leaf trace or branch trace is involved in its 
formation. It contains parenchyma and through it the cortex 
and pith are confluent. (See Fig. 84.) 
SECRETION SAcs (SECRETION CELLS) 
These were formerly parenchyma cells which sooner or later 
lost their protoplasm and nucleus and became receptacles for 
oil, resin, oleoresin, mucilage or some other secretory substance. 
They are generally found in parenchyma regions of stems, roots, 
leaves, flower or fruit parts and frequently possess suberized 
walls. Good illustrations of these structures may be seen in 
Ginger and Calamus. 
INTERCELLULAR AIR SPACES 
Intercellular air spaces are cavities filled with air found 
between cells or groups of cells throughout the bodies of higher 
plants. Their function is to permit of the rapid movement of 
atmospheric gases through the entire plant body. They are 
formed either by the breaking down of the middle lamella of 
the cell walls, where several cells come together, and a later 
separation of the cells at these places (Schizogenous intercellular- 
air-spaces), or by a breaking down and disappearance of cell 
walls common to groups of cells (Lysigenous intercellular-air-spaces). 
In terrestrial plants which live in middle regions (mesophytes) 
and in desert plants (xerophytes) the intercellular-air-spaces are 
avéragely small and more or less angular. In plants of swamp 
