162 ROOTS 
It has been demonstrated in case of some plants that roots ex- 
crete poisonous substances which tend to impede further root 
activity. These deleterious substances, with those formed 
through the decomposition of roots and other organic matter, may 
be responsible for much of the soil sterility that is so commonly 
attributed to the lack of the necessary mineral salts. In fact, 
some think that the value of fertilizers depends mainly upon their 
neutralizing effect of these deleterious substances. The improve- 
ment of the soil, when fields are allowed to lie fallow, is, at least, 
partly due to the disappearance of these deleterious substances 
through oxidation. It seems that in many cases the deleterious 
substances are more poisonous to the roots of plants of the same 
kind, and this may help explain the value of crop rotation. 
Water, Air, and Parasitic Roots 
Water Roots. — When branches of some herbaceous plants are 
cut off and set in water, roots develop from the submerged por- 
tion. Branches of the Geranium and 
Wandering Jew root readily in water and 
will grow for a long time in ordinary 
river or well water. The twigs of Willows 
Hie: to — ems will develop water roots when set in 
floating water plant, which water. Willows, growing on the edge of 
has only water roots. ponds and streams, develop roots which 
Slightly magnified. After penetrate the soil and also roots which 
Stevens: dangle in the water. There are a number 
of small Seed Plants, like the Duckweeds, which float on the 
surface of the water and have no roots other than water roots. 
(Fig. 142.) 
Air Roots. — Some plants depending upon soil roots also de- 
velop air roots. The brace roots of Corn are at first air roots and 
later enter the soil. Some climbing plants, like the Poison Ivy, 
develop air roots which attach the plant to the support. Many 
Orchids and some plants of the Pineapple family grow supported 
on other plants and have only air roots. The Tillandsia, called 
Spanish Moss, although not a Moss at all, is very common in 
southern regions, growing on the branches of trees with its roots 
dangling in the air. 
Air roots differ in structure from soil roots. Air roots, unless 
