348 Development and [BOOKII. 



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. Nageli 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 Nageli 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 ; 



