Re: What would happen to a plant if there was xylem but no phloem?

Nonvascular plants, such as mosses, liverworts, and hornworts, are limited in 
size because they lack both xylem and phloem so must make do with just 
cell-to-cell transport. Having xylem and not phloem or phloem and not xylem 
would probably not be workable for land plants. Xylem mainly moves water and 
mineral nutrients from the roots to the shoots. The roots depend on phloem for 
their survival because they need sugars from the leaves for energy. Removing the 
phloem in a ring at the base of a tree trunk (girdling) allows the shoots to 
survive because the xylem provides them with water and mineral nutrients via the 
xylem. However, the roots eventually starve because they cannot get sugars from 
the shoots via the phloem. 

You might suspect that submerged aquatic plants really don't need xylem because 
they are surrounded by water. However, phloem would be needed if the aquatic 
plants have roots to anchor them, as they often do, and long stems with leaves 
near the surface where the light is the greatest. Submerged aquatic plants 
benefit from xylem because the roots can absorb more mineral nutrients from the 
soil than the stem and leaves can from the water.