In these brown types with bodies of considerable thickness (Laminariaceae and Fucaceae), there is, however, a further differentiation of the internal tissues.
Cells of this type are often called trumpet-hyphae (though they have no connection with the hyphae of Fungi), and in some genera of Laminariaceae those at the periphery of the medulla simulate the sieve-tubes of the higher plants in a striking degree, even (like these latter) developing the peculiar substance callose on or in the perforated cross-walls or sieve-plates.
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 many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary meristem.
This leptom is not so highly differentiated as in the most advanced Laminariaceae, but shows some of the characters of sieve-tubes with great distinctness.