ON TREE TWIGS.[34]
284. The speaker's purpose was to exhibit the development of the common forms of branch, in dicotyledonous trees, from the fixed type of the annual shoot. Three principal modes of increase and growth might be distinguished in all accumulative change, namely:—
1. Simple aggregation, having no periodical or otherwise defined limit, and subject only to laws of cohesion and crystallization, as in inorganic matter.
2. Addition of similar parts to each other, under some law fixing their limits and securing their unity.
3. Enlargement, or systematic change in arrangement, of a typical form, as in the growth of the members of an animal.
285. The growth of trees came under the second of these heads. A tree did not increase in stem or boughs as the wrist and hand of a child increased to the wrist and hand of a man; but it was built up by additions of similar parts, as a city is increased by the building of new rows of houses.
Any annual shoot was most conveniently to be considered as a single rod, which would always grow vertically if possible.
Every such rod or pillar was, in common timber trees, typically either polygonal in section, or rectangular.
If polygonal, the leaves were arranged on it in a spiral order, as in the elm or oak.