A specialized conducting tissue of this kind, used mainly for transmitting organic substances, is always developed in plants where the region of assimilative activity is local in the plant-body, as it is in practically all the higher plants.
This is the case in the Fucaceae, and in a very marked degree in the Laminariaceae in question, where the assimilative frond is borne at the end of an extremely long supporting and conducting stipe.
In the liverworts we find fixation of the thallus by water-absorbing rhizoids; in certain forms with a localized region of water-absorption the development of a primitive hydrom or water-conducting system; and in others with rather a massive type of thallus the differentiation of a special assimilative and transpiring system.
In the more highly developed series, the mosses, this last division of labor takes the form of the differentiation of special assimilative organs, the leaves, commonly with a midrib containing elongated cells for the ready removal of the products of assimilation; and in the typical forms with a localized absorptive region, a well-developed hydrom in the axis of the plant, as well as similar hydrom strands in the leaf-midribs, are constantly met with.
The root differs from the shoot in the characters of its surface tissues, in the absence of the green assimilative pigment chlorophyll, in the arrangement of its vascular system and in the mode of growth at the apex, all features which are in direct relation to its normally subterranean life and its fixative and absorptive functions.