ature, nor has their wood time to
solidify and ripen. Such stems therefore commonly die back from the top
in winter, or at least all their upper buds are small and feeble; so the
growth of the succeeding year takes place mainly from the lower axillary
buds, which are more mature.

63. =Deliquescent and Excurrent Growth.= In the former case, and
wherever axillary buds take the lead, there is, of course, no single
main stem, continued year after year in a direct line, but the trunk is
soon lost in the branches. Trees so formed commonly have rounded or
spreading tops. Of such trees with _deliquescent_ stems,--that is, with
the trunk dissolved, as it were, into the successively divided
branches,--the common American Elm (Fig. 80) is a good illustration.

[Illustration: Fig. 80. An American Elm, with Spruce-trees, and on the
left Arbor Vitae.]

64. On the other hand, the main stem of Firs and Spruces, unless
destroyed by some injury, is carried on in a direct line throughout the
whole growth of the tree, by the development year after year of a
terminal bud: this forms a single, uninterrupted shaft,--an _excurrent_
trunk, which cannot be confounded with the branches that proceed from
it. Of such _spiry_ or _spire-shaped_ trees, the Firs or Spruces are
characteristic and familiar examples. There are all gradations between
the two modes.

Section V. ROOTS.

65. It is a property of stems to produce roots. Stems do not spring from
roots in ordinary cases, as is generally thought, but roots from stems.
When perennial herbs arise from the ground, as they do at spring-time,
they rise from subterranean stems.

66. =The Primary Root= is a downward growth from the root-end of the
caulicle, that is, of the initial stem of the embryo (Fig. 5-7, 81). If
it goes on to grow it makes a _main_ or _tap-root_, as in Fig. 37, etc.
Some plants keep this main root throughout their whole life, and send
off only small side branches; as in the Carrot and Radish: and in
various trees, like the Oak, it takes the lead of the side-branches for
several years, unless accidentally injured, as a strong tap-root. But
commonly the main root divides off very soon, and is lost in the
branches. _Multiple primary roots_ now and then occur, as in the
seedling of Pumpkin (Fig. 27), where a cluster is formed even at the
first, from the root-end of the caulicle.

[Illustration: Fig. 81. Seedling Maple, of the natural size; the root
well supplied with