SHOOTS 
85 
1 i 
Stems which increase in diameter mostly belong to the 
great groups called Dicotyledons and Conifers. To the 
former belong most of our common trees, such as maple, 
oak, beech, hickory, etc. (see Figs. 58, 59, 60, 61), as 
well as the great majority of common herbs ; to the latter 
belong the pines, hemlocks, etc. (see Figs. 56, 57, 64, 
193, 194). This annual increase in diameter enables the 
tree to put out an increased number of branches and 
hence foliage leaves each year, so 
that its capacity for leaf work be- 
comes greater year after year. A 
reason for this is that the stem is 
conducting important food sup- 
plies to the leaves, and if it in- 
creases in diameter it can conduct 
more supplies each year and give 
work to more leaves. 
59. Monocotyledons. In other 
stems, however, the vessels are 
arranged differently in the central 
region. Instead of forming a hol- 
low cylinder enclosing a pith, they 
are scattered through the central 
region, as may be seen in the cross- 
section of a corn-stalk (see Fig. 
80). Such stems belong mostly to a great group of plants 
known as Monocotyledons, to which belong palms, grasses, 
lilies, etc. For the most part such stems do not increase in 
diameter, hence there is no branching and no increased 
foliage from year to year. A palm well illustrates this 
habit, with its columnar, unbranching trunk, and its crown 
of foliage leaves, which are about the same in number from 
year to year (see Figs. 81, 82). 
60. Ferns. The same is true of the stems of most fern- 
plants, as the vessels of the central region are so arranged 
that there can be no diameter increase, though the ar- 
FIG. 80. A corn-stalk, showing 
cross-section and longitudinal 
section. The dots represent 
the scattered bundles of ves- 
sels, which in the longitudinal 
section are seen to be long 
fiber-like strands. 
