340 VEGETABLE PHYSIOLOGY 
special epidermal development, or give rise to dense masses 
of root-hairs. In the first case, which is common among 
epiphytic orchids, the epidermis is many cells thick, and 
is known as the velamen. The cells are small tracheids, 
with curious reticulated or spiral thickenings, and are often 
perforated. These peculiar tracheids contain only air, and 
the velamen has consequently a curious glistening greenish 
appearance. The mass‘of tracheids forms a kind of spongy 
covering to the root, and is capable of condensing and 
absorbing aqueous vapour from the moist atmosphere which 
usually surrounds it. At other times when the air is dry and 
there is a danger of evaporation from the root, this velamen 
acts as a protective membrane against loss of water in this 
way. The second case is illustrated by many aroids, and 
the dense plexus of root-hairs borne upon the aerial roots 
serves the same purpose as the velamen of the orchids. 
Besides these roots, thus adapted to absorb watery vapour 
from the air, epiphytes frequently have others which are 
closely applied to the surface of the bark on which they 
are growing. These are often strap-shaped, and cling very 
closely to the tree, absorbing from the bark the soluble 
products of its decomposition and any mineral débris that 
may be accidentally carried thither. The small amount 
of such food stuffs available will explain the relatively large 
development of the root system, which is in much greater 
proportion than in ordinary terrestrial plants. 
Parasites are another class of plants that have under- 
gone much modification of structure in consequence of 
their mode of life. The parasitic habit is seen most com- 
pletely in the group of Fungi, but it is by no means con- 
fined to them. We find many cases of partial or complete 
parasitism among flowering plants. In all cases we notice 
that the parasitic habit is associated with a degeneration 
of structure, which especially affects the vegetative organs. 
The fungus which is parasitic in habit derives all its 
nourishment from the plant or animal whose tissues it has 
invaded. Other plants of the same group are not parasitic, 
but live upon decomposing organic matter, being known as 
