4 THE PALM-STEM. 



discovery necessarily excited the greatest attention ; the 

 doctrine, that the vascular bundles of the Monocotyledons 

 originate in the centre of the stem, and press the older 

 bundles outward ; that this process continues until the 

 older and solidified vascular bundles form a layer at the 

 circumference of the stem sufficiently firm and hard to 

 resist the pressure of the younger; that thereupon all 

 further increase of thickness of the stem must cease, and 

 that from this results the columnar form of the stem this 

 doctrine appeared to explain the peculiarities of the growth 

 of Monocotyledons in a manner so simple and satisfactory, 

 that it not only passed into all text-books, but was even 

 used by De Candolle for the systematic division of vascular 

 plants into Endogens and Exogens. The observations of 

 Dupetit-Thouars * showed, indeed, that the stem may 

 grow to an unlimited thickness in many Monocotyledons ; 

 they were not, however, any more than the later obser- 

 vations of Mirbel, sufficient to shake the belief in the 

 correctness of Desfontaines' doctrine, and only gave oc- 

 casion for the assumption that in some Monocotyledons a 

 second peripherical growth occurs, in addition to the 

 central vegetation. One voice, alone, but therefore the 

 more important, declared against the theory of the central 

 growth of Monocotyledon s.f Moldenhawer stated that in 

 the stem of Phcenix dactylifera a line of division occurs, 

 on both the outside and inside of which liber-bundles 

 are equally developed; that around those in the latter 

 situation, spiral vessels are produced subsequently to their 

 first formation, and they thus become ligneous bundles ; 

 and that of these ligneous bundles, the inner run to the 

 older leaves, and the outer to the younger; that con- 

 sequently, in other words, the date-palm has a periphe- 

 rical growth. Like so many other admirable remarks of 

 this exact observer, this proposition was so completely 

 neglected by other vegetable anatomists, that not one 

 thought it even worth the trouble of mentioning. 



* Premier Essai sur la Vegetation. 



f Beitrage zur Anatom. d. Pflanzen, 53. 



