. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. TRANSITION FROM STE:M TO ROOT protoxj'lem-strands are directly continuous, wiiilst the metaxylem is de\'eloped towards the exterior in the stem and towards the interior in the root, and occupies a more or less intermediate position in the region { the hj'pocotji) where the transition from the one to the other type of structure takes place. The phloem is similarly continuous, and the transition from root to stem mainly involves a lateral and outward displacement accom- panying the enlargement of the vascul
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. TRANSITION FROM STE:M TO ROOT protoxj'lem-strands are directly continuous, wiiilst the metaxylem is de\'eloped towards the exterior in the stem and towards the interior in the root, and occupies a more or less intermediate position in the region { the hj'pocotji) where the transition from the one to the other type of structure takes place. The phloem is similarly continuous, and the transition from root to stem mainly involves a lateral and outward displacement accom- panying the enlargement of the vascular cylinder (Fig. 42). The transition between the two organs is, however, often very complex, being accom- panied b}' sphtting or union of conducting strands, as a result of which the number of protoxylem-groups in stem and root do not necessarily corre- spond. The dcAxTopment of the different regions of the axis from the gro^ving points has already been described in Chapter II, but some additional details as to the manner of origin of the vascular tissues ^\ill serve to ampUfy the picture. These first appear in the plerome a short distance behind the tip as so-called procambial strands, composed of very narrow, elongated, thin-walled cells with pointed ends, dense protoplasmic contents, and prominent nuclei. They develop by repeated longitudinal di\dsions in the ceUs of the plerome, in which transverse diidsion almost ceases at an early stage, the narrow segments thus formed subsequently elongating. In the stem each procambial strand gives rise to a vascular bundle, whilst in the root there is usually a single strand from which the whole vascular cylinder is differentiated. The inner- most elements, in each of the procambial strands of the stem, become the spirally thickened protoxj'lem-vessels, whilst simul- taneously the ouferiTjost elements develop into the first-formed. Fig. 42.—Diagrams showing one type of transition from the vascular structure of the root to that of th
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