GROUPS OF TISSUES, OR TISSUH-SYSTEMS. 49 
is that the tissues have a concentric arrangement: the tra- 
cheary tissue is encircled by a layer of parenchyma; this 
by one of sieve-tissue; this again by fibrous tissue; and 
80 On. 
90. De Bary’s classification of fibro-vascular bundles is 
useful in designating their general plan. He includes all 
forms under three kinds, viz., (1) the Collateral bundle, 
which has one mass of xylem by the side of a single mass 
of phloém; (2) the Concentric bundle, which has its tissues 
arranged concentrically around one another; (8) the Radial 
bundle, which has its tissues arranged radially about its 
axis. 
91. The development of the fibro-vascular bundle takes 
place in this wise: in the previously uniform primary 
meristem there arises an elongated mass of cells, consti- 
tuting the Procambium of the bundle; as it grows older 
the cells, which were at first alike, become changed into 
the vessels, fibres, and other elements of the bundle-tissues. 
In most higher flowering plants this change begins on the 
two sides of the bundle—i.e., on the outer edge of the 
phloém and the inner edge of the xylem; from these points 
the change into permanent tissue advances from both sides 
toward the centre of the bundle. 
92. In some cases all of the procambium is changed into 
permanent tissue, forming what is termed the closed bun- 
dle; in other cases there is left between the phloém and 
xylem a narrow zone of the procambium (now called the 
cambium), forming what is known as the open bundle. 
Closed bundles are thus incapable of further growth, while 
open bundles may continue to grow indefinitely. 
93. The fibro-vascular bundles of leaves and the repro- 
ductive organs are quite generally reduced by the absence 
