1. A system for transferring information in a computer network from a server to a client computer, the information including a plurality of hierarchically related objects, wherein a viewable subset of the objects is displayed on a display device connected to the client computer in the form of a navigable tree having expandable nodes, the viewable subset being visible in a navigation pane on the display device, the system comprising: a tree descriptor array comprising information describing each of the objects to be displayed in the navigation pane; and a tree descriptor string comprising information describing a hierarchical structure of expanded nodes in the tree; wherein the tree descriptor array and the tree descriptor string are sent from the server to the client computer; and wherein the tree descriptor string comprises a list of only those said nodes which are to be expanded and displayed on the display device.