462 



FOURTH GRO UP. SEED-PL A NTS. 



roots occurs in Monotropa Hypopitys, 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 1 . 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-plants 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 (Hippitris, Aldrovanda, Ceratophyllum 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 2 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 Iberis, 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 

 stem, till they anastomose below with deeper 

 lying bundles. The leaf-trace-bundles do 

 not bend deeply inwards into the tissue of 

 the stem but turn downwards and run parallel 

 to one another and at the same distance 

 everywhere from the surface of the stem, so 



FIG. 405. Samtitais Ebnlus. Leaf-trace bundles in two ' . 



that the layer in which they lie is con- 

 centric with it ; this layer appears in the 

 transverse section as a ring dividing the 



bundles. Besides these are slenderer bundles s" s" united fundamental tisSUC ItttO pttJl and primary 

 together by horizontal brandies, from which bundles nn ascend ,. ,- > f j .1 



info the stipules. After Hanstein. cortex. 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, the primary 

 medullary rays. If there is no secondary growth in thickness, no further change takes 

 place ; but usually even in annual stems (Heltanthus, 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 cambium 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-ring through the formation 

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



internodes ; they lie in a cylinder which lias been spread out 

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

 each leaf receives from the stem a middle bundle h /( and 

 two strong lateral bundles / s' ; the descending bundles divide 

 below and their limbs enter the intervals between the lower 



1 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 Fiats Carica, they are arranged 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. 



