348 Development and [BOOK ir.

portance in phytotomy, because it is the only way to the under- standing of secondary growth in thickness in true woody plants. It was noticed above, that von Mohl had proved in 1831 the separate character of the bundles which begin in the stem and bend outwards into the leaves where they end, so that the entire system of bundles in a plant consists of single bundles isolated when formed and subsequently brought into connection with one another. N�geli had already examined the correspond- ing circumstances in the vascular Cryptogams in 1846, when Schacht took the retrograde step of making the vascular system in the plant originate in repeated branching, instead of in subsequent blending of isolated strands ; Mohl declared unhesitatingly against this mistake in 1858, but it was refuted at greater length and still more clearly by Johannes Hanstein in 1857, and by N�geli in 1858. Hanstein in a treatise on the structure of the ring of wood in Dicotyledons confirmed Nageli's previous statements, and proved in the case of Dicotyledons and Conifers that the first woody circle in the stem is formed from a number of vascular bundles, which are identical with those of the leaves and originate in the primary meristem of the bud. These primordial bundles pass down- wards through a certain number of internodes in the stem independent and separate, and either retain their isolation to the point where they end below or unite with adjacent bundles which originated lower down. Hanstein happily termed the portions of the vascular bundles, which enter the stem from the base of the leaf and traverse a certain portion of it in a downward direction, leaf-traces, so that it may be stated briefly, that the primary wood-cylinder in Dicotyledons and Conifers consists of the sum of the leaf-traces. Nageli's observa- tions were of a more comprehensive character, and supplied, as we have seen, a terminology for tissues. He distinguished three kinds of vascular bundles according to their course ; the common bundles, which represent Hanstein's leaf-traces in the

stem, and whose upper ends bend outwards into the leaves