28 ROOTS 



Roots of Plants Require More or Less Air. — The securing 

 of this air is facilitated in many of our cultivated plants by 

 drainage and by cultivation of the soil, thereby keeping it loose 

 so that the air penetrates it readily. But plants growing in a 

 state of nature have various parts and organs through which the 

 necessary air is secured. Many aquatic plants, such as pond 

 lilies and cat-tails, have air passages by which the air passes from 

 the parts exposed to the air to the submerged parts. The swamp 

 cypress has peculiar upward root growths known as " knees," 

 which project above the surface of the water and serve for the 

 absorption of oxygen. Many forest trees have root growths 

 extending above the surface of the soil which serve for the 

 same purpose. The filling in around trees which is frequently 

 necessary in making grades is injurious because it prevents the 

 air from reaching the roots. 



EXERCISES WITH ROOTS 



1. Direction of Growth of Roots. — Sprout a nvimber of seeds and 

 suspend in a moist chamber on lon^- ])ins or by means of threads so that 

 the root tips will point in different directions. Examine from day to day 

 and note the direction of growth. Small moist chaml>ers can be made by 

 putting a small amount of water in a wide bottle or! glass jar; run a hat 

 pin through tlie cork for supporting seeds. 



2. Direction of Growth in Soil. — Sprout a number of seeds and plant 

 in wet sand in such a manner that the root tips point in different direc- 

 tions. Examine one a few days later and note the direction of growth 

 of roots and stems. 



3. Growth Against Resistance. — Partly fill a small cup with mer- 

 cury. Pour a little water on the surface. Fasten a germinated seed in such 

 a manner that the root tip rests on the mercury. Inclose in a moist cham- 

 ber for 24 hours. Has the root ti]) penetrated the mercury? Does this 

 require force? 



4. Geotropism or Direction of Root Growth. — Sprout a number of 

 beans in sphagnum moss or other loose material that will insure the forma- 

 tion of straiglit roots. Mark the roots into short, equal sections, using 

 India ink. pin them to corks and place in large moist chambers * in such 



* Moist chambers can be made by lining large Ijattery or other glass 

 jars with wet filter paper and covering so as to prevent excessive 

 evaporation. 



