Roots are used to uptake water (among other things) from the soil, therefore the closer proximity to the exterior by the xylem provides a more efficient means of transporting water through the epidermis and into the vascular tissue.
hi, because in aerial part transportation of sap is more important.
Hi Junho,
I am curious: where did you find that: xylem in roots: outside of bundle? I attach some pictures which all show that for the root vascular bundle, the xylem is inside and the phloem is outside.
1. The xylem typically lies adaxial with phloem positioned abaxial. In a stem or root this means that the xylem is closer to the center of the stem or root while the phloem is closer to the exterior. In a leaf (as you mentioned, 'shoots'--where the leaves develop), the adaxial surface of the leaf will usually be the upper side, with the abaxial surface the lower side. This is why aphids are typically found on the underside of a leaf rather than on the top, since the sugars manufactured by the plant are transported by the phloem, which is closer to the lower surface. [from Wikipedia]
2. adaxial: On the side that is towards the axis or central line, usually on the upper side
3. See the three vascular bundles below: (1) Leaf, (2) Stem (sunflower), and (3) Roots
4. When you draw a picture, the vascular bundles from leaves to stems, the 'upper part' xylem in leaf v. b. will automatically position towards to 'the center' of stem or further into roots. Seems to me, there is no 'cross' as you described.
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