ANGIOSPERMS.—DICO TYLEDONS. 



463 



rays, and the consequent bridging over of the intervals between the separate layers 

 of the fascicular cambium (Fig. 407). The cambium-ring thus formed produces layers 

 of phloem on the outside and layers of 

 xylem on the inside, and is itself constantly 

 increasing in circumference ; all the tissue 

 formed by the cambium-ring on the side of 

 the cortex may be styled secondary pJiloem, 

 all the xylem formed on the inside scconda/y 

 xylevi^ in opposition to the primary phloem 

 and the primary xylem of the leaf-trace- 

 bundles which were in existence before 

 the formation of the cambium-ring. While 

 the wood produced by the cambium-ring 

 forms a hollow cylinder, the xylem of the 

 primary bundles projects like ridges into 

 the pith, and often gives it the form of a star 

 in the transverse section ; the whole of 

 this protoxylem is included under the 

 term medullary s/ieath, and we may in 

 like manner with Nageli designate as a 

 cortical sheath the whole of the phloem 

 of the primary bundles within the primary 

 cortex. The medullary sheath and the 

 cortical sheath, being masses of tissue 

 that are in existence before the formation 

 of the cambium-ring, grow in length with 

 the elongation of the internodes, and 

 therefore consist of elements usually of 

 considerable length; the medullary sheath 

 has annular, spiral and reticulated vessels 

 with long members intermixed with long 

 wood-fibres, while the phloem-bundles of 

 the cortical sheath, which have become 

 separated further from one another by 

 the increase in circumference of the stem, 

 contain long bast-fibres, often much thick- 

 ened but still flexible, with or without 

 long cambiform cells and long-membered 

 sieve-tubes. The elements of the second- 

 ary cortex and secondary wood which are 

 formed from the cambium-ring are shorter ; 

 in the latter there are no annular and 

 spiral vessels, and instead of these we now 

 find shortly-membered broader vessels 

 with bordered pits, and round them 

 wood-fibres with wood-parenchyma inter- 

 mixed. The secondary phloem forms 

 either repeated layers of thick-walled 

 bast-fibres and masses of thin-walled 

 partly parenchymatous phloem, or only 

 the latter, or the two mixed up together 

 in a great variety of ways. The primary 

 cortex with the epidermis is ultimately superseded in most cases by formations of 

 periderm and bark, though they sometimes keep pace with a considerable growth in 



Fig. 406. Clematis viticcUa. Extremity of branch reiulcrcd 

 transparent by removal of the outer surface and treatment with 

 potash ; the leaves decussate. The two younKest pairs arc still 

 without leaf-trace-bundles ; three bundles p>iss into each of the 

 older leaves (which are removed). The leaf-trace is three- 

 bundled. Tlie central ones of these bundles (a, d, g, k. x, <) 

 traverse one internode, split into two limbs in the next node and 

 attach themselves by them to the lateral bundles of the pair of 

 leaves belonging to that node (see e.g. g and i). The two lateral 

 bundles also' run through one internode, converge as they bend 

 outwards at the next node, and alt.ich themselves to the s.nmc 

 lateral bundles to which the limbs of the middle bundles .irc 

 united. After Nageli. 



