THE BOOT AND THE SOIL 59 



tance from the original root. Lateral roots originate in 

 the internal tissues of the stem or root and not close to 

 the surface, as do buds (131). 



88. Moisture excites root growth. Roots develop, 

 as a rule, from parts of the stem that are maintained 

 for a certain time in contact with abundant moisture. 

 In the pumpkin vine and tomato plant above mentioned, 

 nearness to the soil furnishes a moist atmosphere. A 

 corn stalk pegged down to the ground for some distance 

 will usually root at all joints of the stem in contact with 

 the soil. A potato plant grown under a bell-jar, where 

 the air is nearly saturated with water, will form roots 

 at any joint of the stem. In parts of the tropics where 

 the air is very moist, certain plants, as orchids and the 

 Banyan tree (Ficus indica), emit roots freely from the 

 stem above ground. Cuttings (358) and layers (349) 

 form roots because they are maintained in contact with 

 abundant moisture and at a suitable temperature. Cut- 

 tings of some plants, as the willow and nasturtium, root 

 promptly when their stems are immersed in water. 



89. Oxygen is necessary to the life of roots. Since 

 the cells of newly-formed roots are filled with proto- 

 plasm, they must have access to the oxygen of the air, 

 or they can neither live nor grow. This is shown by a 

 simple experiment. Boil a quantity of water fifteen 

 minutes or longer, to exhaust it of free oxygen, and then 

 cool it quickly by setting the dish containing it in cold 

 water. Now place a healthy cutting (358) of some 

 plant that roots freely in water, as willow, nasturtium 

 or wandering jew (Tradescantia), in each of two tumblers. 

 Pour a part of the cool, boiled water into one of the 

 tumblers and add a little olive oil to form a film over the 

 liquid, thus preventing it from absorbing more air. Then 



