PLANT TISSUES 83 
(a) Concentric, with xylem central in bundle. Seen in stems and 
leaves of nearly all ferns and the Lycopodiacece. 
( b ) Concentric, with phloem central in bundle. Seen in stems and 
leaves of some Monocotyledons. Examples: Calamus and Conval- 
laria rhizomes. 
V. Radial, characterized by a number of xylem and phloem masses 
alternating with one another. Seen in the roots of all Spermalo- 
phytes and Pteridophytes. 
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 libers. 
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 ( Scliizogenous 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 ( xeropkytes ) 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. 
