1. A method of embedding a collective network in a parallel computer system organized as a torus network, said computer system including a multitude of nodes, each of the nodes having a plurality of receivers and a plurality of senders, the method comprising: adding to the torus network a central collective logic to route messages among at least a group of said nodes in a tree structure, wherein, at defined times, one of said group of nodes is a root node and the others of said group of nodes are leaf or intermediate nodes, and including said central collective logic forming separate up-tree and down-tree paths for routing messages among said group of nodes; the central collective logic selecting one of the up-tree path and down-tree path for one of the messages for each sender depending on a collective class map of said one of the messages; routing messages from the leaf or intermediate nodes to the root node in an up tree direction; processing the messages being routed from the leaf or intermediate nodes to the root node to form a processed message; and sending the processed message back from the root node to at least one of the leaf, intermediate or root nodes.