SHOOTS 



85 



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. 



