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 



p IG 142 Lemna a w ^ 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 



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 



