24 THE ECOLOGY OF ROOTS 



nursery stock compact root systems have been induced by frequent 

 transplanting;. At each transplanting the longer roots are cut off 

 and thus at the final and ])ermanent transplanting there is a mini- 

 mum of disturbance of the root system. Even large trees may be 

 successfully transplanted- if their preparation for it is begun a year 

 or two in advance. The preparation consists of gradually cutting 

 off the roots a few feet from the base of the tree and loosening the 

 soil near the tree so that a large number of short absorbing branches 

 are produced. When the tree is finally transplanted these branches 

 are ready to begin at once the work of absorption. 



Of very great importance in connection with transplanting is the 

 maintenance of a reasonable balance between root and shoot. If a 

 considerable proportion of the root system has been lost in trans- 

 planting the shoot must be trimmed back in order that water may 

 not be lost faster than it can be absorbed by the roots. On the 

 other hand, if the shoot is trimmed back too much, growi:h will be 

 retarded because the reduced leaf surface will prevent a sufficient 

 amount of food manufacture to supply the needs of the more exten- 

 sive root system. 



13. Water Roots.— Roots of floating plants which grow entirely 

 in water are much less extensively developed than are soil roots. 

 There are usually few if any branches and no root hairs. Some 

 floating plants have no roots at all, absorption taking place through 

 stems or leaves. Sahinia, the water fern, for example, has two 

 kinds of leaves, broad flat ones which float on the surface, and 

 elongated, much dissected ones which hang down into the water 

 like roots and perform the function of absorption. The duckweed, 

 Wolffia, although it is a seed plant, has neither roots, stems, nor 

 leaves but consists of a small football-shaped thallus through the 

 entire surface of which absorption may take place. Lemna, another 

 duckweed, has a single unbranched root only a few millimeters in 

 length, while Spirodela has several such roots. 



Water plants which have roots extending into the soil are inter- 

 mediate between ordinary soil plants and floating plants in the 

 relative development of their root systems. When the roots of such 

 plants are growing in water they ordinarily remain unbranched, 

 but if they extend into the soil they branch freely and produce root 

 hairs like ordinary soil roots. 



14. Air Roots.— x\ir roots, like water roots, are relatively un- 

 branched and without root hairs. Some climbing plants, such as 

 poison ivy and Virginia creeper, produce innumerable air roots 



