PLANT PHYSIOLOGY 145 
minates, and the radicle penetrates the bark, and sends 
out haustoria which fuse with the xylem of the host 
plant. This plant is a perennial, and each year new 
‘“sinkers”’ are developed, which penetrate the new wood 
in the host. 
Viscum capense and V. rotundifolium live on various 
South African trees. Loranthus and Striga are partial, 
Hydnora a total, parasite. 
120. Saprophytes.—Saprophytes are plants which 
obtain their food from decaying organic substances. 
Many orchids are saprophytes, either partial or total. 
The roots or rhizomes of such saprophytic plants exhibit 
a curious formation, known as MycorruizA. ‘The 
whole root-tip is covered by a fungus sheath, the hyphae 
of which penetrate into the root-cells, and also spread 
out in the soil around. This fungus appears to absorb 
organic food from the decaying vegetable matter in the 
soil, and convey it to the plant, and the fungus at the same 
time obtains food from the plant. This is an arrange- 
ment known as SYMBIOSIS i.e. a union between two 
plants, the fungus and the flowering plant, in which 
each derives benefit from the other. Many plantsin the 
orders Orchidaceae and Ericaceae exhibit this mycor- 
rhiza. 
A curious relation between roots and bacteria exists 
in the order Leguminosae. On the roots of plants of 
this order are found tubercles, and these are caused by 
certain bacteria, which penetrate through the root- 
hairs into the cortex of the root. These bacteria have 
the power of absorbing free nitrogen from the air 
of the soil, and passing it on to the host in the form of 
nitrates, while they themselves obtain carbohydrates 
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