This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
When a seedling of a shrub or tree has completed its first year's growth, it usually terminates in a bud, covered with scales. If, on the resumption of growth next spring, this bud continues the growth of the stem or axis, the latter would be monopodial, and would continue to be so from year to year, so long as this order of growth is maintained. The Conifers are good examples of monopodial stems, particularly the species of Pinus, Abies, Picea, Sequoia, and Arau-caria. If the leaders of such trees get broken, or eaten off by animals, or killed by frost, they rarely recover themselves by the production of a new, upright axis. Araucaria excelsa is a rare exception to this rule. If the leader is cut off for the purpose of propagation, one or more upright shoots on the old stock will result, and these may be utilized in the same way. The side branches are useless, because they do not grow into true tree shape, even though they produce roots. The side branches are secondary monopodia, continued by growth at the apex, but they always retain the same relation to the primary axis. Trees of this character may often be improved by stopping the growth of too rampant lateral shoots, but not the leader.
Sympodial stems are very numerous, and are not the result of the continuous growth of the primary axis from year to year. For instance, a seedling Vine would be monopodial until it produces a tendril, which is the termination of the primary axis. A bunch of flowers or berries is the equivalent of the tendril, and in any of these cases the axis is continued by the growth of an axillary bud or axis that pushes the primary one to a side. It will be noticed that the tendril or bunch of flowers is always opposite to a leaf, not in its axil. A similar method of growth may be noted in the Tomato. The first bunch of flowers is really the termination of the axis of the seedling plant. An axillary bud grows strongly and pushes the bunch of flowers on one side, and in its turn terminates in a bunch of flowers, and so on indefinitely. The number of bunches on a single-stemmed Tomato is an index to the number of axes thus superposed, and forming the "sympodium" or combination of several separate axes. This mode of growth is brought about in the Willows by the dying of the terminal bud at the end of each season, and as a result of it the growth next year must be continued by an axillary bud. Sympodial branching may be observed in the Lilac and Horse-chestnuts. Whenever a stem or branch ends in a bunch of flowers, the axis afterwards dies back to the first pair of buds, which will produce two new axes, if they are leafy buds, but if flower buds, then growth must be continued from a lower pair of buds.