Ubik wrote:Now I'm wondering how do plants communicate the knowledge of the orientation from the tips of the roots to the leaves and the upwards-growing part in general?
While your question was already answered by Angua, it is interesting to know that plants do communicate both internally and with other plants. They can communicate from the roots to the leaves and vice versa, as well as in any other direction imaginable. They have three main ways to do it.
The first way is hormones through vessels. Cells at point A put hormones in a vessel (either rootwards phloem or leavewards xylem), which takes it downstream to point B, where other cells respond to it. An example of this is auxin, which is sent from the tips of shoots through phloem. It tells shoot buds along the way to "wait". This is why a shoot will start branching out sooner if you remove the tip. In fact, this is probably the most important purpose of auxin.
The second way is hormones through air, also known as pheromones. This is a common way for branches of a plant to communicate with other branches, both of the same plant and of other plants. It has been shown that when you lit a branch of a plant, other nearby plants will show a stress response, too (yes, plants can have stress). Incidentally, this is also the reason why putting a banana in a fruit basket may help the other fruits to ripen.
The third way is direct cell-to-cell signaling. Nerve cells are famous for this because they can do it very fast and over long distances, but all cells can do it and plant cells are no exception. This is how Mimosa pudica (shy plant) folds its leaflets when you touch it, and how Venus fly traps can close their graspers at the right time.