PLANT TISSUES 
119 
Xylem is that part of a fibro-vascular bundle that contains wood 
cells and fibers. It may also contain tracheae, tracheids, seldom 
sieve tubes. 
Phloem is that part of a fibro-vascular bundle that contains sieve 
tubes, phloem cells, and often bast fibers. . 
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 intercellu¬ 
lar-air-spaces are averagely small and more or less angular. In 
plants of swamp or marsh habit they are medium-sized, while in 
those which live entirely in the water (hydrophytes) they are of large 
size and more or less rounded. 
SECRETION RESERVOIRS 
These structures are either found as globular or irregular spaces, 
as in Orange and Lemon Peel and Eucalyptus leaves, containing oil 
or oil and resin when they are called internal glands, or, as tube-like 
spaces filled with hydrocarbon principles such as are found in Pine 
leaves an,d stems, when they sometimes receive the name of secretion 
