462 



FOURTH GROUP.— SEED-PLANTS. 



roots occurs in Monotropa Hypopttys, and peculiar modifications of the roots like the 

 organs of adhesion of the Podostemaceae occur in the parasitic Dicotyledons ; hence 

 we may conclude that in the Podostemaceae we have to do with retrogressively 

 metamorphosed forms. 



Histology \ As regards the histology of the Dicotyledons I confine myself here to 

 some account of the behaviour of the vascular bundles and of secondary growth in 

 thickness. 



With the exception of a few water-plant of simple structure in which an axile 

 vascular cylinder runs through the stem and is constantly developed at its apex 

 as a cauline bundle, with which the bundles of the leaves which are of later formation 

 afterwards become connected {Hippiiris, Aldfovanda, Cejatophyllwn and also Trapa 

 partially), the general rule in Dicotyledons is that common bundles are first 

 formed and their ascending limbs enter the stronger foliage-leaves usually in some 

 numbers, where they run through the leaf-stalk and midrib beside - but usually separate 

 from one another and give off bundles to form the venation in the lamina. The limbs 

 which descend in the stem, the leaf-trace-bundles, generally pass downwards through 

 several internodes, pushing in between the upper portions of older leaf-trace-bundles 

 and sometimes dividing (Fig. 405), and ultimately laying themselves alongside the 



older bundles lower down and coalescing with 

 them. In this process each bundle in the 

 stem becomes twisted, as in Ibe>-is, and 

 always towards the same side, so that the 

 bundles from leaves of different heights, 

 which form a sympodium by their coalescence, 

 ascend in a spiral line within the cortex ; 

 but they often run parallel to the axis of the 



S'^l't'^^Hli^x^'t^f^l^YT'ClflrVl^f'^ stem, till they anastomose below with deeper 



lil \\\f m n iill \ I If ly'i^g' bundles. The leaf-trace-bundles do 



11 I Iff H '1 I '^'^^ bend deeply inwards into the tissue of 



I I In I I I I the stem but turn downwards and run parallel 



llil lllll 111 11 l|l' III "I to one another and at the same distance 



FIG. 403. sambucus Ebuius. Leaf-trace bundles in two everywhere from the surface of the stem, so 



internodes ; they lie in a cylinder which has been spread out ^hat the layer in which theV lic is COn- 

 in a plane ; each internode bears two opposite leaves, and . •' . •' 



each leaf receives from the stem a middle bundle /I A and CentHc With it; this layer appears in the 



two strone lateral bundles / s' ; the descending: bundles divide , . . -,..,. , 



below and their limbs enter the intervals befween the lower tranSVCrSC SeCtlOn aS a ring dividing the 



t'oTe1h";byWi::n:^i":a:cheff;t:hiXtn^^^ fundamental tissue into pith and primary 



into the stipules. After Hanstein. covtex. The portions of the fundamental 



tissue which lie between the bundles appear 

 in the transverse section as rays connecting the pith and the primary cortex, \h^ primary 

 Diedullajy 7-ays. If there is no secondary growth in thickness, no further change takes 

 place ; but usually even in annual stems {Helianthiis, Brassica, etc.), and always in stems 

 and twigs of more than one year and which become woody, growth in thickness begins 

 after the elongation of the internodes. A layer of cainbiiim forms in each bundle between 

 the phloem which lies on the outside and the xylem which is nearer the axis of the stem, 

 and these layers, which lie side by side in a ring and are at first separated by the medul- 

 lary rays between the bundles, unite into a closed cambium-ritig through the formation 

 of iiiterfascicitlar cambium by divisions in the intermediate cells of the medullary 



* De Bary, Vergl. Anatomic. 



^ When several bundles enter a leaf-stalk, they are usually separated from one another by a con- 

 siderable breadth of fundamental tissue ; but sometimes, as in Ficus Carica, they are ariangcd in the 

 transverse section of the leaf-stalk in a circle and form a closed hollow cylinder, which divides the 

 stalk into pith and cortex Isolated bundles also run through the medullary portion of the leaf-stalk 

 of Ficus, as in the stems of some Dicotyledons. 



