In the work just mentioned Bernhardi treats of other forms of tissue as well as vessels, and endeavours to distinguish and classify them more exactly than had hitherto been done. He contrasts favourably with his contemporaries in the fact, that he sought to define the histological terms employed as precisely as possible, a great step in advance at a time when phytotomic conceptions were in a very misty condition. He distinguishes three chief forms of vegetable tissue, pith, bast, and vessels.
By pith he means the tissue which Grew had named parenchyma, and which is still so called; it remained a question with him whether the cells of the pith are pierced by visible pores. By the word bast he understood not only the fibrous elements of the rind, but those of the wood also, and in general what is now known as prosenchyma; this agrees very well with Malpighi's view, which was adopted by Bernhardi and by all his contemporaries, that the inner layers of the bast are changed into the exterior layers of wood to make the increase in thickness of the woody stem; but he did not admit the same origin in the case of the innermost portion of the wood, for this is formed from the first in the young shoots, which alone contain true spiral vessels with threads that may be wound off.
Bernhardi distinguishes vessels into two main groups, air-vessels and vessels properly so called. He calls the first group air-vessels for the same reason that led Grew to give them that name, namely, that they are filled with air during a part at least of the period of vegetation; they are found in the wood, and, where there is no closed woody body, there the woody bundles are formed both of vessels and also of bast strands which enclose vascular canals. These latter he next divides into three chief kinds; annular vessels, which he was the first to discover, true spiral vessels with a band which can be unwound, and scalariform vessels, by which term he understood not only those with broad slits, such as are found in Ferns, but also the pitted vessels in secondary wood. His idea of annular and spiral vessels was perfectly correct, and he
