ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 
293 
(unreliable) or not. The first kind (annular and spiral vessels) are 
characterised by the thin parts of the wall running continuously round 
the vessel, while the thicker parts are not connected longitudinally ; 
lienee the membrane is capable of longitudinal stretching, and the 
thickened portion may, under certain circumstances, become detached. 
In the second kind, the thin portions of the membrane are not continuous, 
while the thickened parts are connected in all directions into a network 
(reticulate and pitted vessels). The character of the perforations is 
always the same, whether in spiral, annular, or reticulate tracheids ; 
there is either one large round bordered pit, or there are a number of 
narrow bordered pits arranged in a scalariform manner. 
The unrolling of the thickening-bands does not take place, as has 
hitherto been stated, by the rupture of the unthickened portions of the 
membrane, but by the detachment of the thickening-bands from the thin 
membrane, the latter remaining quite intact. “ Reduced vessels ” may 
be formed in two ways : — by the attachment of the thickening-bands by 
their greatest breadth, in other words, by the replacement of the bordered 
pits by simple pits ; or by the incomplete development of the thickening- 
bands and their looser arrangement. 
The author insists also on a uniformity in the physiological purpose 
of all kinds of vessel. The thin portions permit the passage of water 
between the vessels and the surrounding elements ; while the thickened 
portions enable them to resist radial pressure. 
Modifications of the Primary Cortex in Dicotyledons.* — It has 
long been known that in dicotyledonous stems the central cylinder may 
become sensibly thicker without splitting the cortex. This has generally 
been explained by assuming that the cells of the cortex multiply by radial 
septation. M. Eberhardt finds the process to be a more complicated 
one. Taking the ash as a type — from which other trees differ only in 
immaterial points — the total diameter of the stem does not increase with 
that of the central cylinder, the cortex being proportionately reduced in 
thickness. While septation takes place in the inner layers of the cortex, 
this is not the case with the outer layers ; the lacunae at first formed in 
them disappear as the result of the internal pressure ; and finally a con- 
siderable number of the cells themselves are destroyed. It is probable 
that a similar process takes place also in herbaceous stems. 
Secondary Increase in the Primary Vascular Bundles of Mono- 
cotyledons.! — M. C. Queva finds, in a stem of Dioscorea illustrata, a kind 
of cambial zone formed before the completion of the vessel in the middle 
portion of the stem, which, however, is soon destroyed by the growth of 
the surrounding elements, not advancing to the formation of secondary 
xylem and phloem. In the tubers of Gloriosa superba he now finds also 
vascular bundles which have a distinct cambial zone, not present in the 
earliest primordium, but forming eventually secondary xylem and sieve- 
tubes. 
Placental Fibro vascular Bundles of Primula.^ — It is often stated 
that in the vascular bundles which run into the placental column of 
* Comptes Rendus, cxxviii. (1899) pp. 463-6. 
t Ass. Frany. pour l’avanc. d. sciences, 1897. See Bot. Centralbl., Beih., viii. 
(1899) p. 295. % Comptes Rendus, cxxviii. (1899) pp. 259-61. 
