of the Stem of the Phanerogamia, 393 



added, the vascular bundles of the first hollow cone are developed 

 on through the following, as far as the circumference, where 

 they enter leaves or buds. They therefore form an arc convex 

 internally, the length and convexity of which depend upon the 

 form of the terminal bud. 



Since all the new portions, whether these be in the form of 

 thickenings of the old vascular bundles in the Dicotyledons, or 

 the rudiments of new vascular bundles in the Monocotyledons, 

 are constantly formed on the outside of the primary vascular 

 bundles, the older and deeper-seated bundles running from the 

 axis to the periphery of the leaves and buds must necessarily 

 cross the younger bundles ascending higher in the axis, or the 

 cambial structures which have been formed from within outwards. 



In reviewing Schleiden's explanation of the development of the 

 stem, we recognize above all, as the great merit of it, the state- 

 ment that, as has long been known of the Dicotyledons, so also 

 in the Monocotyledons, the origin of the vascular bundles occurs 

 in a cambium-layer situated under the surface of the stem, in 

 which is developed, simultaneously with the bundles, the paren- 

 chymatous tissue destined for the further completion of the stem. 

 This explanation forms a strong contrast to the views of Mene- 

 ghini, who believed that the vascular bundles were formed in the 

 well-developed parenchymatous tissues through the influence of 

 currents of sap. The correctness of the account given by 

 Schleiden has been confirmed by all subsequent researches. 



We cannot speak so favourably of the other parts of this 

 theory, since Schleiden gives far too little weight to the diver- 

 sities of internal organization corresponding to the systematic 

 position of plants, and attributes far too much value to the ex- 

 ternal form of stems, especially to the length of their internodes, 

 and ascribes their outward variations to internal differences of 

 development which do not exist in nature. 



That the Monocotyledons and Dicotyledons cannot be sharply 

 separated from each other in respect to the internal structure of 

 their stems — a series of intermediate structures existing — has 

 long been known ; yet, for all that, the distinction is very clearly 

 marked in the great majority of cases. Schleiden only pays 

 attention to the growth in thickness of the Dicotyledonous bun- 

 dle, and the absence of this in the Monocotyledons. But this 

 very point is well known to present many exceptions; it is 

 therefore important to keep in view the second great distinction 

 — the internal, convex, curved course of the vascular bundles of 

 the Monocotyledons. 



That I directed especial attention, in my ' Anatomy of Palms/ 

 to this course of the vascular bundle, and to the difference of 

 structure which one and the same bundle exhibits in different 



