ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
439 
stelar arrangement is a closed tube with external and internal endoderm, 
but no internal phloem. 
Origin of the Vascular Elements in the Growing Point of the 
Root of Monocotyledons.* — Sig. R. Pirotta and Dr. L. Buscalioni point 
out that in the root of Monocotyledons the plerome is always well differ- 
entiated from the other elements of the growing point, and gives birth to 
the central cylinder of the root and to the ground tissue, which becomes 
differentiated into peripheral pericambium and central parenchyme. 
Within the mass of ground tissue stand the vascular elements, either 
isolated or united into groups or series, generally simple sieve- and 
vascular bundles, arranged in a circle towards the periphery of the 
central cylinder. These vascular elements always originate directly 
from the differentiation of one or more cells of the meristem of the ple- 
rome, thus constituting another important histological difference between 
Monocotyledons and Dicotyledons. 
Vascular Bundles in the Receptacle of the Labiatse.t — M. L. Vidal 
has studied the course of the fibrovascular bundles in the receptacle of 
Lamium maculatum. The position of the abortive posterior stamen is 
clearly marked in the vascular plane of the flower, and may be repre- 
sented by a phloem-bundle. The vascular system of the carpels is 
perfectly symmetrical in reference to the median plane of the flower. It 
is composed, for each carpel, of a median bundle proceeding from two 
primitive bundles situated right and left of the median plane, either in 
the cauline vascular ring ( Lamium ), or in the pith ( Phlomis ), and of two 
placental bundles, each of which is from the first fused with its homo- 
logue of the other carpel and with that of a lateral stamen. 
Formation of Sieve-Tubes in the Roots of Monocotyledons.^ — From 
observations on a large number of species of Monocotyledons, M. G. 
Chauveaud arrives at the following general conclusions : — In all the 
examples studied, primary sieve-tubes are formed, which may be re- 
placed by others, or may themselves persist. These first sieve-tubes are 
formed by the septation of a mother-cell, which gives rise to the sieve- 
tube and to its sister-cell. The sieve- tube is lozenge-shaped (Gramineae, 
Oyperaceaa, &c.) or pentagonal ( Naias , Potamogeton , Vallisneria, &c.), 
according to the inclination of this septum to the plane passing through 
the axis of the sieve-tube. When the first sieve-tube is replaced by 
others, these latter are usually formed directly at the expense of those 
previously existing. There may be only one of these directly formed 
sieve-tubes ( Avena , Triticum , &c.), or there maybe several. In this case 
either all the cells composing the phloem-island develop into sieve-tubes 
<( Zea Mays, Scilla italica, Narcissus poeticus, &c.) ; or a certain number 
only, the remainder persisting in the state of jdiloem-cells ( Anthurium 
crystallinum, Monstera deliciosa , &c.). When the primary sieve-tubes are 
not supplanted by others, either the phloem-bundle is reduced to this 
primary sieve-tube and to three or four cells adjacent to it (Juncus 
balticus, Pontederia cordata , Hydrocharis morsus-ranse ), or to the primary 
•sieve-tube and its sister-cell (Naias, Potamogeton, Vallisneria'). 
* Atti R. Accad. Lincei, vii. (1898) pp. 60-2. 
t Journ. de Bot. (Morot), xii. (1898) pp. 46-52. 
x Ann. Sci. Nat. (But.), iv. (1897) pp. 307-81 (6 pie.). 
