BVNDLE-SrSTEM IN THE LEAVES. 399 



parts which will all be included under the term leaf-expansions, the : bundles dis- 

 tribute themselves along the surface, sometimes ending free, sometimes anastomosing 

 with one another in a reticulate manner. 



The bundles, especially those in the flattened expansions, lie as a rule in the 

 protrusions or ridges of the surface, which are known as nerves, ribs, or veins. The 

 course of these, the nervation or ribbing, and that of the vascular bundles often 

 exactly coincide, both phenomena are therefore usually indicated by the same name. 

 There is no further objection to this convenient use of the term ; but it must be 

 pointed out that two different phenomena are thus dealt with, one belonging to the 

 external formation of the parts, and referring to the relief of the surface, and another, 

 which refers to the inner structure ; and that though the two phenomena are always 

 closely correlated, they do not coincide always, or in all points. 



The phenomena of surface-relief as seen in coarse ribbing may be here 

 assumed as known, the reader being referred to the literature on this subject, 

 especially to that of Pteridography and Palaeontology ^. In treating of the arrange- 

 ment of the vascular bundles we must take into consideration (i) the divarication of 

 the bundles, that is their course in the direction of the surfaces of the leaf-expansions 

 (Sect. 91); (2) their position — as seen in a transverse section — within the other 

 tissues (Sect. 92). 



Sect. 91. As regards the divarication of the bundles it must be premised that 

 for the middle portion (Rachis, Petiolus communis) of compound and deeply-divided 

 leaves, and for the main ribs of many, especially of large leaves, the same rules 

 hold good as have been above stated for the course of the bundles in the petiole. 

 The bundles are continued from a petiole, which contains several bundles, into the 

 main nerve, and here are arranged in a channel open upwards, or in one or several 

 circles, and are connected by anastomoses one with another : in their course towards 

 the periphery some of them pass out into the branches of the nerves, others give off 

 branches into them, while they decrease in number and size proportionately. The 

 stronger lateral nerves of a lamina may also contain several bundles, e. g. Quercus 

 pedunculata. A. B. Frank (/. c.) has given an exact description of the course of the 

 bundles of the leaf in this plant, and notes on the same in other plants. 



The bundles which pass into the foliar expansion, either as branches from the 

 above-named parts, or directly from the nodes, either remain unbranched, or give off 

 branches, often up to high orders, their strength diminishing as a rule with each 

 higher order, but in a degree which varies greatly in each individual case. 



The bundles and branches of whatever order either end free in the foliar 

 expansion, or unite and anastomose with others. 



Free ends sometimes lie at the periphery of the foliar expansion, in flat leaves 

 especially at the margin and point, occasionally also at the surfaces : sometimes they 



' As chief works and sources of infonnation the following may here be cited : L. von Buch, 

 Ueber die Blattnerven, and Die Gesetze ihrer Vertheilung, Monatsbr. d. Berliner Academie, 1852, 

 p. 42.' — C. von Ettingshausen, Die Blattskelete der Dicotyledonen, Wien, 1861, fol. ; and the following 

 articles of the same authorfrom the Sitzungsberichten (S.) and Denkschriften (D.) der Wiener Academie : 

 Apetalae (D. XV) ; Papilionace^ (S. XII) ; Bombaces (D. XIV) ; Celastrinese (D. XIII) ; Euphor- 

 biacese (S. XII); Loranthacese (D. XXXII); Graminese (S. LII. l). — For the Ferns compare 

 Mettenius, Filices Horti bot. Lipsiensis. 



