248 PRIMARY ARRANGEMENT OF TISSUES. 



trace of the next lower pair. The bundles of the trace of successive pairs all pectinate 

 with one. another. Besides this, other coalescences must occur at or below the node, 

 since in the internodcs investigated the transverse section showed only eighteen bundles 

 instead of twenty as assumed according to the scheme, and as actually occurs in Gn. 

 Gnemon. 



On WeKvitschia comp. Chap. XVI. 



To avoid repetition, the Cycadece will also be described in the chapter cited. 



II. Anomalous Dicotyledons. 



Sect. 62. A not inconsiderable number of Dicotyledons, some Cvcadeae, and 

 Welwitschia differ in their bundle-system from that which characterises their aUies, 

 in that the primary bundles are not arranged in a simple ring. Either they contain 

 a ring of bundles arranged according to the usual type, while additional bundles are 

 found either within these, that is in the pith, or outside them, that is in the outer 

 cortex ; or the bundles are arranged in several, often not sharply distinguished circles, 

 or so arranged that they appear in transverse section irregularly scattered between 

 other tissues, with the exception of the most peripheral bundles, which may be 

 distinguished as a ring well marked off from the outer cortex. 



These more or less remarkable exceptions to the main type either occur in 

 quite isolated species in typically formed genera and families (e.g. Umbelliferse), or 

 in numerous species of otherwise typically formed genera (e.g. Begonia), or they are 

 characteristic of certain genera, or small families (e.g. Nymphaeaceae, Calycanthacese, 

 Podophyllum, Diphylleja), more rarely even of large families, as Piperacese and 

 ]Melastomaceas. But even in the latter, exceptions occur to that grouping of the 

 bundles which holds for the majority of their allies. 



The above phenomena are based either upon an oblique radial course of the 

 bundles of the leaf-trace, or upon the appearance of caulvie bundles besides the bundles 

 of the trace which are arranged in the typical ring. Discounting the Nyctaginese, 

 which will be described below (Chap. XVI), many Amarantacese, &c, with medul- 

 lary bundles, the following cases belong to this category. 



a. Medullary bundles. 



I. All bundles belong to the leaf- trace : some on entering the stem are arranged 

 in the typical ring, and have a radially perpendicular course in it : others, passing 

 further inwards, are therefore medullary, and are either scattered through the pith or 

 arranged in rings. To this class belong most Cucurbitacece, species oi Amararitus 

 and Euxolus, Phytolacca dioica, the Piperacece, doubtless also the herbaceous Ber- 

 ber idece. Podophyllum, Diphylleja, Leotitice ; further species of Papaver Thalictrum, 

 and Actcca. 



The bundles of the climbing Cucurbitacex'^ (Cucumis, Cucurbita, Bryonia, Tladiantha, 

 Cyclanthera pedata) are arranged in two rings (Ecbalium elaterium, which has no tendrils, 

 has only one circle of bundles) : those of the outer ring are opposite the corners of the 

 stem, and are of equal number with these, e. g. five in Cucumis sativus, Cucurbita, Tladi- 

 antha dubia, Cyclanthera pedata, seven in Bryonia dioica : those of the inner ring alternate 



' Bernharcii, BcoLacht. iiber Pflanzengefasse, p. 20. — Sanio, Botan. Zeitg. 1864, p. 227. — 

 Nageli, I.e. p. 77. 



