. Practical botany. Botany. 50 PEACTICAL BOTANY pulls, but laterally they are supported by the earth. Accordingly it is usual to find young roots with a fibrous central cylinder of comparatively small di- ameter, surrounded by a coating of much weaker tissue (Fig. 34, C). 47. Limited thicken- ing of annual stems. In stems of large dicoty- ledons which die to the ground every year, such as sunflowers, ironweeds, hemp, giant ragweed, and so on, growth in thick- ness goes on tliroughout the summer. The outer cells of the cambium con- tinually split up, by the formation of tangential partitions (p
. Practical botany. Botany. 50 PEACTICAL BOTANY pulls, but laterally they are supported by the earth. Accordingly it is usual to find young roots with a fibrous central cylinder of comparatively small di- ameter, surrounded by a coating of much weaker tissue (Fig. 34, C). 47. Limited thicken- ing of annual stems. In stems of large dicoty- ledons which die to the ground every year, such as sunflowers, ironweeds, hemp, giant ragweed, and so on, growth in thick- ness goes on tliroughout the summer. The outer cells of the cambium con- tinually split up, by the formation of tangential partitions (parallel to the bark), and so form new layers of bark. The iirner cells of the cam- bium, in a similar way and to a still greater extent, form new wood, and thus tlie stem goes on increasing in thick. ness. But in sueh plants as thc)se just mentioned the activity of the cam- bium is strictly limited. After it has given rise to a certam amount of new tissue, growth stops and the stem dies down to the ground. The death of annual stems in the autumn is often thoughtlessly supposed to be due to the. Tig 35 Cio&s sec tion of a stit k of oak wood m, medullaiy rays, running from bark U> pith', r, "annual rings"; b, lumndaries bt't^\'een "" rings," ]ii)rous from iireseiire of many ducts; (', interior fibrous layers of dead liai-k ; pi, hard plates of dead bark, splitting away from each other but attached to bark beneath. Reduced. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bergen, Joseph Y. (Joseph Young), 1851-1917; Caldwell, Otis William, 1869- joint author. Boston, New York [etc. ] Ginn and company
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