VEGETATIVE ORGANS OF PLANTS. 259 



leaves for themselves. Plants like the plum are, there- 

 fore, 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 divide into two 

 seed-leaves or Cotyledons, and whose stems increase 

 externally by addition of new rings of growth the 

 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, espec- 

 ially at first, the lateral roots issue from the tap-root 

 with great order and regularity, 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 develop- 

 ment. In many cases the tap-root grows to a great 

 length, and forms the most striking feature of the radi- 

 cation 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 

 (Rumex), and in seedling fruit trees. The upper por- 

 tion of the tap-root of the beet, turnip, carrot, and rad- 

 ish expands under cultivation, and becomes a fleshy, 

 nutritive mass, in which lies the value of these plants 

 for agriculture. The lateral roots of other plants, as of 

 the dahlia and sweet potato, swell out at their extremi- 

 ties to tubers. 



Crown Roots. Monocotyledonous plants, or Endo- 

 gens, i. e., plants whose embryos have only one seed- 

 leaf, or Cotyledon, and whose stems do not increase by 

 external additions, such as the cereals, grasses, lilies, 

 palms, etc., have no single descending axis or tap-root, 

 but produce crown roots, i. e., a number of roots issue 

 at once from the base of the stem. This is strikingly 

 seen in the onion and hyacinth, as well as in maize. 



