ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 317 



Primary Vascular Elements in Roots of Monocotyledons.* — R. 

 Pirotta publishes an abstract of bis paper, which will soon be printed 

 in full with plates in the Annuario del R. Istituto Botanico di Roma, 

 on the origin and differentiation of the primary vascular elements in 

 the root of Monocotyledons. Nageli and others have stated that the 

 formation of the vessels and their lignification is centripetal ; but they 

 do not agree as to the mode of origin of the vascular elements. There 

 are two kinds of vessels, distinct in origin and structure : the vascular 

 rays and the central vessel or vessels. Several authors have admitted 

 the centripetal origin of the vascular rays. Russow found that from 

 the plerome are formed a peripheral pericambium, which yields the 

 vascular rays and sieve-tubes, and a medullary parenchyma. Van 

 Tieghem at first regarded the central vessels as belonging to the vascular 

 rays, but more lately has admitted that in some cases they may be of 

 medullary origin. Cerulli-Irelli and Chauveaud have established that 

 the central vessels have no connection with the vascular rays. Nicolai 

 found that in some Gramineae the central vessels appear before the rays, 

 but are the last to lignify. The author now states that the apical 

 plerome differentiates into three histogens : — the pericambium, the pro- 

 cambial parenchyma, and the central parenchyma. In the second of 

 these the vascular rays are developed in regular order alternating with 

 the sieve-tubes. In the central parenchyma arise the central vessels, 

 one, a few, or several, without relation to the vascular rays. They are 

 the first to appear, and develop close to the apex of the plerome, and 

 in a centrifugal order if they be several in number. The vascular rays 

 develop more slowly (and always after the sieve-tubes) and centri- 

 fugally, the largest inside and the smallest outside. But lignification 

 takes place centripetally — first the small, then the large vessels of the 

 vascular rays, and finally the central vessels. This is the general 

 rule. 



Tubercle of Tamus communis. f — L. du Sablon supplements in a 

 short paper the previous work of Bucherer on the morphology of the 

 tuber of black bryony. The young tuber has at its base a growing 

 point and grows vertically downwards with strong positive geotropism. 

 The tubercle of the seedling has below the epidermis a few cortical 

 layers, the cells of which divide radially and not tangentially, and a 

 central cylinder in which cell-division takes place in all directions. A 

 cork cambium is formed at an early stage in the sub-epidermal layer, 

 and in the pericycle a meristematic ring by which growth in thickness is 

 provided for exactly as in Dracaena. The direction of the vascular 

 bundles is very variable. The cork cambium goes over the growing 

 point which becomes therefore covered with a cork layer ; but in the 

 meristematic ring, as we approach the growing point, cell-division be- 

 comes very irregular, and at the growing point itself occurs in all direc- 

 tions as in the apical meristem of a stem. In its exogenous origin, and 

 internal structure the tuber suggests a stem, though the absence of 

 leaves and vertical downward growth are root-characters. 



* Atti dclla E. Accad. dei Lincei, Eendiconti, xi. (1902) pp. 49-52.' 

 t Revue Gen. Botan., xiv. pp. 145-50 (9 figs.). 



June 18th, 1902 Y 



