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 aérial 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 aérial 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 
become to all intents 
and purposes the stem 
of the fig tree, which 
now leads an independ- 
ent life. 
“89. The root system. 
— The entire mass of 
roots belonging to a 
plant, with all its rami- 
A fications and subdivi- 
Fie. 91.— Root system of a tobacco plant. sions, composes 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, 
