phloem tubes to not need to be as thin (or dead for that matter), because they do not rely on capillarity for movement, as w/xylem cells. and the only active transport involved in the movement in the phloem is actually loading the sugars into the phloem...the rest is all diffusion, which is passiveOriginally posted by Joe C.
I agree with survivor, i think it's C.
It's not really about The overall area of 'M' and 'W', look at the diameter of the tubes. This should clearly show that M is xylem tissue, as one of the mechanisms which carries water up the tubes is capillary action - the thinner the tubes are the better this works. Phloem tubes do not need to be as thin, as movement is by active transport.
Also, 'W', is an area of great variation as to tube diameters, this shows the larger tubes, which can carry the food up or down the plant due to active transport, much smaller tubes in amongst these are indicative of companion cells, which are necessary for the 'source to sink' mechanism.