What do the trees eat?
To start off with, plants for the vast majority of their life cycles do not eat. They are considered autotrophs, and receive their nutrients through photosynthesis.
The part of the tree that "gathers" the nutrients are the leaves. The leaves contain chlorophyll which is used in photosynthesis. Through photosynthesis Salix alba receives all of the nutrients it needs (for the most part there are certain exceptions to this rule). Then it encounters the problem of how to transport these nutrients to the rest of the tree.
The phloem of this cross section is labeled P. The xylem is labeled X. This picture was taken by Ryan R. Mackenzie
This is where phloem comes in handy. Phloem is the part of plant that is used to transport nutrients gathered in the leaves to the rest of the plant. The phloem is found in the most inner part of the bark. This is the reason why buckling a tree, cutting a circle all the way around the tree through the bark, will kill the tree. It will starve to death because it is lacking the means to transport the nutrients.
In addition to phloem, is another type of transport plants used called xylem. This type is different in phloem in two main ways. First, xylem is composed of dead cells while phloem is still living. Also, xylem is used to transport water versus how phloem transports nutrients.
Next lets look at reproduction.