79 



'continuous wood on the inside of the ring, produces only new 

 areas of fundamental tissue with isolated vascular bundles. 



74. In the young roots of Mono- R oa ts. 



cotyledons, Dicotyledons, and Gymnosperms the arrangement 

 of the primary vascular bundles is different from that found in 

 the stem. The xylem and phloem strands of each bundle 

 separate from each other as they pass into the root and are 

 there arranged side by side on different radii of the root and 

 the bundles are then said to be radial. The xylem strands 

 also become twisted on themselves so that the protoxylem, 

 instead of being internal, as in the stem, is external. 

 Sometimes the xylem strands meet in the centre of the root 

 and sometimes they do not, there then being a central 

 pith. These separate strands of xylem and phloem follow a 

 straight longitudinal course in the root instead of curving as 

 do the bundles of the stem. In the roots of Dicotyledons and 

 Gymnosperms secondary growth in thickness occurs simul- 

 taneously with the similar growth in the stem. In the roots 

 cambium layers first appear on the inside of the phloem strands 

 and, extending thence laterally on both sides, they eventually 

 coalesce opposite the xylem strands, and thus form a complete 

 -cambium ring. This jing gives rise to xylem elements on the 

 inside and phloem elements on the outside, as in the stem, 

 broad medullary rays being usually formed opposite the primary 

 -xylem strands. The wood of an old root is usually more 

 porous than that of the stem but otherwise they very closely 

 Tesemble each.~oth.er in the possession of annual rings and other 

 characters. The cambium of the root is a continuation of 

 that of the stem, there thus being an uninterrupted cambial 

 layer throughout the stem and root and their branches. 



In those Monocotj'ledons which exhibit secondary growth 

 a, cambial layer may arise in the root outside the primary 

 hurdles but it only produces closed vascular bundle strands 

 scattered in fundamental tissue, as in the stem. 



75. In addition to the typical Abnormal 



mode of secondary growth above described, cases of abnormal Deveiop- 

 development are not uncommon among Dicotyledons and mentt 

 Gymncsperms. In Cocculus laurifolius, Bauhinia Vahlii, Cycas, 

 and several other plants, the cambium ring first formed ceases 

 to grow alter a time and then a second cambium ring arises 

 -outside the bast formed by the first. This also ceases to grow 

 after a limited time and a third ring arises, and so on, the 

 stem in consequence exhibiting very characteristic, more or 

 less concentric, bands of wood and soft bark-like tissue, which 

 have been already mentioned on page 18. 



