much misinterpretation and to gross errors on the part of later writers. Malpighi thought he observed a peristaltic movement in these vessels, a delusion to which many of the nature-philosophers were particularly fond of surrendering themselves at the beginning of the present century.
In addition to the bundles of fibres and the tracheae, Malpighi observed a number of tubes in Ficus, Cupressus, and other plants, which allowed the escape of a milky juice, and he concludes that similar special tubes might be present also in the wood of stems from which milk, turpentine, gum, and the like exude.
Such are the elementary organs of plants, as far as they were known to Malpighi; in the subsequent part of his book we find them applied to a histology of the stem, and here a mistake at once makes its appearance, which, resting on his authority, was reproduced by the phytotomists of the 18th and even of the early part of the 19th century,—the theory, namely, that the young layers of wood in the stem originate in the periodic transformation of the innermost layers of bark (secondary bast-layers); Malpighi was led into this mistake, as it appears, partly by the softness and light colour of the alburnum, partly by its fibrous character. In this substance the spiral tubes are gradually formed, and as the mass becomes more solid and compact, it subsequently forms the true wood.
The pith lies in the centre of the stem, and, according to Malpighi, consists of numerous rows of spheres ('multiplici globulorum ordine') arranged longitudinally one after another, and composed of membranous tubes, as may be clearly seen in walnut, elder, and other trees. In this place also he mentions the milk-vessels in the pith of the elder. Passing over many and various matters, it may be mentioned next that Malpighi recognises the connection of the layers of tissue in young shoots with those of the parent-stem, and very expressly notices the same continuity of structure between the leaf and the axis of the shoot. He then briefly touches on the anato-
