78 PRACTICAL COURSE IN BOTANY 



ishment from the air, they are generally subsidiary to soil 

 roots, like the long dangling cords that hang from some 

 species of old grapevines ; or they subserve other purposes 

 altogether than absorbing nourishment, as the climbing 

 roots of the trumpet vine and poison ivy. A very remark- 

 able development of aerial roots takes place in the "stran- 

 gling fig " of Mexico and Florida, which begins life as a small 

 epiphyte, from seeds dropped by birds on the boughs or 

 trunks of trees. When it gets well started, the young plant 

 sends down enormous aerial roots, which find their way to 

 the ground, and in time so completely envelop the host that 

 it is literally strangled to death (Plate 3, p. 73). When this 

 support is removed, the sheathing roots take its place and 



t become to all intents 

 and purposes the stem 

 ^ of the fig tree, which 



_^ . . '^-'^"^6^^ -^X.^-^ ^ now leads an independ- 



Sf V^7h?C^"^^ ^9- The root system. 



— The entire mass of 

 roots belonging to a 

 plant, with all its rami- 

 fications and subdivi- 



FiG. 91. — Root system of a tobacco plant. gions, COmpOSCS a rOOt 



system. The extent of root expansion is in general about 

 equal to that of the crown, thus bringing the new and 

 active parts under the drip of the boughs where the moisture 

 is most abundant. Some plants have root systems out of 

 all seeming proportion to their size. A catalpa seedling 

 six months old showed, by actual measurement, 250 feet 

 of root growth, and it is estimated that the roots of a thrifty 

 cornstalk, if laid end to end, would extend a mile. In the 

 development of the root system, a great deal depends upon 

 external conditions. In a poor, dry soil, the roots have to 

 travel farther in search of a livelihood, and so a larger system 

 has to be developed than in a more favorable location. 



