THE VEGETATIVE ORGANS OF PLANTS. 237 
true roots by the imbricated buds, of which indications 
may usually be found on their surfaces, e. y., the eyes of 
the potato-tuber. The side or secondary roots are indeed 
marked in their earliest stages by a protuberance on the 
primary root, but these have nothing in common with the 
structure of true buds. The onion-bulb is itself a fleshy 
bud, as will be noticed subsequently. The true roots of 
the onion are the fibers which issue from the base of the 
bulb. The roots of many plants exhibit no buds upon their 
surface, and are incapable of developing them under any 
conditions. Other plants may produce them when cut off 
from the parent plant during the growing season. Such 
are the plum, apple, poplar, and hawthorn. ‘The roots of 
the former perish if deprived of connection with the stem 
and leaves. The latter may strike out new stems and 
leaves for themselves. Plants like the plum are, therefore, 
capable of propagation by root-cuttings, i. e., by placing 
pieces of their roots in warm and moist earth. 
Tap-Roots.—All plants whose seeds readily divide into 
two parts, and whose stems increase externally by addi- 
tion of new rings of growth—the so-cailed dicotyledonous 
plants, or Exogens, have, at first, a single descending axis, 
the tap-root, which penetrates vertically into the ground. 
From this central tap-root, lateral roots branch out more 
or less regularly, and these lateral roots subdivide again 
and again. In many cases, especially at first, the lateral 
roots issue from the tap-root with great order and regu- 
larity, as much as is seen in the branches of the stem of a 
fir-tree or of a young grape vine. In older plants, this 
order is lost, because the soil opposes mechanical hindrances 
to regular development. In many cases the tap-root grows 
to a great length, and forms the most striking feature of 
the radication of the plant. In others it enters the ground 
but a little way, or is surpassed in extent by its side 
brancnes. The tap-root is conspicuous in the Canada 
thistle, dock, (Rumez,\ and in seedling fruit trees. The 
