The identity of the Hadas:
The Torah states that one is to take a Anaf Eitz Avos. This refers to a branch that grows leaves in a chain like braid form which covers most of the branch. This is unlike other branches in which the leaves do not braid or cover majority of the branch. This branch is referred to as the Hadas, myrtle.
Is there a more preferable species of Hadasim? 
There are many species of Hadasim. Some have large leaves and others have short leaves. Some are particular to take only Tzefati Hadassim as they have many small leaves which fully cover the bark. Some Poskim rule that Hadassim with very wide and long leaves is a Hadas Shoteh.
Must the leaves of the Hadas fully cover the bark and reach the leaves above it?
This matter is disputed in Poskim.
 Piskeiy Teshuvos 646/1
 Kashrus Daled Minim p. 118; Piskeiy Teshuvos 646/4