. Botanique agricole, théoriques et pratiques. La botanique, économique, la botanique. Tiges de monocotylédones 137 faisceaux vasculaires de ce rencontré dans les dicotylédones. Au lieu d'être disposés dans un seul anneau, ils apparaissent dispersés dans plusieurs cercles irréguliers dans tout le tissu fondamental (Figs. 69, et 70). Habituellement le cortex est très étroite et peu visibles et d'une moelle distinctes est rarement présent. Les faisceaux sont communs à des feuilles et des tiges comme dans les dicotylédones, mais sur la saisie d'une feuille ils courbent jusqu'à peu à peu près du milieu de la tige, puis à nouveau vers l'extérieur de la courbe en général, enfin de se joindre à d'autres

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. Agricultural botany, theoretical and practical. Botany, Economic; Botany. STEMS OF MONOCOTYLEDONS 137 vascular bundles from that met with in dicotyledons. Instead of being arranged in a single ring, they appear scattered in several irregular circles throughout the ground tissue (Figs. 69 , and 70). Usually the cortex is very narrow and inconspicuous and a distinct pith is rarely present. The bundles are common to leaf and stem as in dicotyledons, but on entering from a leaf they bend gradually inwards to near the middle of the stem, and then generally curve outwards again, finally joining other bundles near the outside of the stem. ve^^ec^o^W In addition to these differences, measurement stcm°'"(Thre^e%i'^s shows that the older parts of such stems which °""^^' "^-J have ceased to grow in length are no thicker than the young parts near the tip; that is to say, the stems of most monocotyledons do not increase in thickness when once they have ceased to grow in length. This incapacity for growth in thickness is due to the fact that the vascular bundles do not possess a cambium tissue, nor is such meristem developed in the ground tissue except in a few special cases which we cannot deal with here. Vascular bundles in which no cambium is present are known as closed bundles. In most grasses the vessels of the wood of each bundle are few in number, and in transverse section appear arranged in the form of a V (Figs. 70 and 71); the vessel nearest the centre of the stem is annular, the others being spirally thickened. Tracheids are not uncommon and thin-walled wood-parenchyma is always present. The bast which lies between the free limbs of the V-shaped wood consists entirely of sieve-tubes and companion-cells. The ground-tissue immediately surrounding each bundle is generally thick-walled and gives mechanical support and pro- tection to the soft parts of the bundle. Similar thickened ground- tissue in larger or smaller amount is met with beneath the epi

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• bookcentury1900
• bookdecade1910
• booksubjectbotany
• bookyear1910