CHAP, in.] of Cell-membrane in Plants. 307 



regarded both as essential constituents of a true vascular 

 bundle. Not less important were his enquiries into the 

 longitudinal course of the vascular bundle in the stem and 

 leaf, which showed that in the Phanerogams the bundles in 

 the stem are only the lower extremities of the bundles, the 

 upper extremities of which bend outwards into the leaves, 

 and that the Monocotyledons and Dicotyledons agree in this 

 particular, though the course of the bundle differs considerably 

 in the two cases. He obtained an important result in this 

 respect in his researches on palm-stems in 1831, when he 

 proved the incorrectness of the distinction between endogenous 

 and exogenous growth in thickness, which had been laid down 

 by Desfontaines, and even employed by De Candolle in fram- 

 ing his system. According to Desfontaines, the wood of 

 Monocotyledons appears as a collection of scattered bundles, 

 of which those that run out above into the leaves come from the 

 centre of the stem. From this very imperfect observation 

 he deduced the view, that the bundles of vessels in Mono- 

 cotyledons originate in the centre of the stem, and that they 

 continue to be formed there, until the older hardened bundles in 

 the circumference form so solid a sheath that they withstand 

 the pressure of the younger ; then all further growth in thick- 

 ness must cease, and hence the columnar form of the mono- 

 cotyledonous stem. This doctrine found general acceptance, 

 and was employed by De Candolle to divide vascular plants 

 into Endogens and Exogens, in accordance with the very 

 general inclination felt in the first half of the present century 

 to distinguish the great groups of the vegetable kingdom by 

 anatomical characters. It is true that Du Petit-Thouars had 

 already shown that some monocotyledonous stems have un- 

 limited growth in thickness ; neither his nor Mirbel's later 

 observations succeeded in shaking the theory, the adherents of 

 which met such cases by assuming a peripherical as well as a 

 central growth. Then von Mohl in the treatise above-mentioned 

 demonstrated the true course of the vascular bundles in the 



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