ANATOMY OF TISSUES. 59 
epidermis. The cells of these layers are similar to those of the 
ordinary epidermis, having no intercellular spaces, containing 
no chlorophyH, and supplied with stomata. The function of 
the lower layers is that of water reservoirs. The best examples 
are the different species of Ficus. 
The secondary epidermis, or periderm, is always preceded by 
the primary epidermis whose office it supplants. Since, there- 
fore, it is a special form of secondary growth, it will be con- 
sidered under that subject. 
4. Vascular and Ground Systems. 
It was formerly customary to consider all the tissues derived 
from the cambium cells as forming a separate system, named, 
from one of its important elements, the vascular system. More 
recent anatomists consider this as only part of a system which 
includes all those tissues whose axial diameter is considerably 
longer than the other two. Following this method of treat- 
ment, we may use the word fibrous tissue to denote the entire 
system. This includes, then, simple and compound strands or 
bundles. Simple bundles are those consisting of one element 
and are either collenchyma, or isolated bast cells, or sieve-tubes. 
Collenchyma occurs chiefly as ribs along the corners of stems 
for the purpose of support, and also as rings or zones extending 
entirely around the stem. Isolated bast cells occur as sub- 
epidermal ribs, in leaves and stems of certain monocotyledons. 
A continuous sheath of bast cells surrounds the aérial roots of 
the epiphytic orchids ; these cells originate from the dermatogen 
and contain air. Isolated strands of sieve-tubes are found in 
the pith of Melastomaceae, and in a large number of plants be- 
longing to other families, monocotyledons as well as dicotyledons. 
Some of the latter have them in the periphery of the pith so near 
the compound bundle as to suggest connection with it. The 
families Myrtaceae, Apocynaceae, Convolvulaceae and others 
