THE VEGETATIVE OEGANS OF PLANTS. 237 



true roots by the imbricated huds, of which indications 

 may usually be found on their surfaces, e. g., 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-cvMings, i. e., by placing 

 pieces of their roots in warm and moist earth. 



Tap-RootSi — All plants whose seeds readily divide into 

 two parts, and whose stems increase externally by addi- 

 tion of new rings of growth — the so-called 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 I'egularly, 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 

 branches. The tap-root is conspicuous in the Canada 

 thistle, dock, {Riimex,) and in seedling fruit trees. The 



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