466 FOURTH GROUP.— SEED-PLANTS. 



either irregularly distributed there or arranged in rings. To this division belong 

 most Cucurbitaceae, species oi Amarattius and Euxohts, Phytolacca dioica, the 

 Piperaceae, etc. Tlie course of the bundles in the Piperaceae is very like that of 

 the bundles in the Commelineae (see under the Monocotyledons). 



2. All the bundles are leaf-trace-bundles, which after entering the stem become 

 a network branching irregularly in every direction. To this division belong 

 the Nymphaeaceae, Gunnereae, Primula auricula and its nearest allies. 



3. The bundles are both leaf-trace-bundles and cauline bundles. The common 

 bundles are disposed in the ring, the cauline are in the pith. To this division 

 belong the Begoniaceae, Orobanchaceae, species of the genus Mamillatia, 

 Melastomaceae, some Umbelliferae and Araliaceae. 



b. Cortical bundles. These are of less frequent occurrence than those of the first group, 

 and are partly leaf-trace-bundles which run for a certain distance outside the typical 

 ring and afterwards bend into it, as in Casuaritia and some Begoniaceae, partly distinct 

 bundles belonging to leaf-traces composed of several bundles, which never enter the 

 ring but form a system of cortical bundles connected with the ring only by anasto- 

 moses in the nodes, as in Calycanthaceae, many Melastomaceae, and other families. 



Besides these different instances of abnormal arrangement of the vascular bundles, 

 there are many cases also of anomalous formation of new tissue. Where the cambium 

 is formed and disposed in the ordinary manner and has the normal persistent activity, 

 an abnormal distribution of the tissue is sometimes found in the zone of the 

 wood and bast. Thus the xylem in the stems of some lianes (Bignoniaceae, 

 Phytocrene) appears to be deeply lobed, and the phloem reaches inwards into the 

 indentations. The phloem in other woody plants has no sieve-tubes ; these are found 

 forming bundles with a soft parenchyma in the xylem, as in some species of Sttychnos. 

 But the formation and arrangement of the cambium, xylem and phloem are sometimes 

 themselves abnormal : — 



a. Besides the normal cambium-ring, a second sometimes appears concentric with it 

 on the inner edge of the xylem, as in Tcconia radicans. 



b. Instead of the one normal cambium-ring in the ring of bundles, several separate 

 portions of cambium make their appearance beside each other round the primary 

 vascular bundles and form partial cambium-rings distinct from the normal general 

 ring, as in the Sapindaceae mentioned above and the Calycanthaceae. 



c. Renewed thickening rings. Growth in thickness begins in the normal manner and 

 then ceases, but is afterwards continued by a new cambium-zone formed in the paren- 

 chyma outside the first one. The process may be repeated and a number of nearly 

 concentric zones be formed. This mode of proceeding, which has already been 

 described in Cycas and Gnetuvi among the Gymnosperms, is found in Dicotyledons 

 in the Menispermaceae and in the stem oi Avicetutia ; in the latter cases all the zones of 

 increase which succeed to the normal one are formed in the primary phloem. In 

 the stems of some lianes {Bauhinia), in Wistaria chinensis and others, the new zones 

 arise in the secondary phloem. 



d. Extrafascicular cambium. The cambium-ring does not pass in the normal manner 

 through the ring of primary vascular bundles, but lies quite outside it, as in the 

 Chenopodiaceae, Amarantaceae, Nyctagineae, MeseinbryaiitJieinum and others 



e. Abnormal dilatation of the inner old parenchyma of the wood usually combined 

 with the formation of new intercalary zones of xylem, phloem and cambium from 

 secondary meristem. To this division belong especially certain lianes {Big/wnia. 

 Caulotretus, etc.) in which the wood is divided into separate portions by dilatation of 

 the parenchyma ; also some fleshy roots, etc. 



With respect to the structure of the vascular bundles of Dicotyledons it is to be 

 observed that the arrangement of the xylem and phloem is usually collateral, the 

 phloem lying on the outside towards the periphery, the xylem inside towards the 

 pith. In the Cucurbitaceae, and some Solanaceae and Apocynaceae, phloem is also 



