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 debris 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 



