ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
545 
arrangement appears to be to combine tbe power of resistance against 
bending with the greatest possible permeability of the membrane for 
water. The so-called unrolling of the thickening-bands of spiral vessels 
does not depend on the rupture of the unthickened portions of the 
membrane, but on the detachment of the bands from the thin membrane. 
The only exception observed to this structure was in the case of Eqni - 
setum, where the thickening-bands are fixed to the wall of the vessel by 
their greatest breadth. 
Transfusion Tissue.* — Mr. W. C. Worsdell discusses the origin of 
this tissue, especially characteristic of the leaves of Gymnosperms. It 
appears to serve as a secondary conducting tissue, and is usually found in 
direct connection with a vascular bundle, extending to the cells of the 
surrounding parenchyme, and consisting of tracheids often accompanied 
by bast-cells. It is universal in the leaves of Coniferas, and nearly so in 
those of Cycadese. From a study of this tissue in the leaves of Salisburia 
adiantifolia , Cycas revoluta, and some other Gymnosperms, the author 
comes to the conclusion that this tissue is a direct derivative of the 
centripetal xylem which normally occurred as an important part of the 
vascular bundle in the ancestors of these plants, but which subsequently 
disappeared. 
Origin and Distribution of Phelloderm. j- — Out of 60 species of 
woody plants examined, Herr F. Kuhla finds phelloderm present in all 
but three. There is no essential difference in this respect between stem 
and root ; but the extent of the tissue varies greatly, not only in different 
species, but even in the same stem. It is independent of the formation 
of periderm, so that abundant cork is by no means necessarily accom- 
panied by abundant phelloderm. With secondary periderm the phello- 
derm usually arises only in small layers. 
The phellogen cells are always of a parenchymatous character, 
though tho maturo phelloderm cells may sometimes assume a prosenchy- 
matous form, the result of a secondary development process. The walls 
of the phelloderm cells usually consist of pure cellulose, but lignified 
membranes are sometimes to be met with. As respects their contents, 
they are characterised by containing chlorophyll where tho light is not 
completely absorbed by the periderm. "Beneath the bark chlorophyll is 
wanting, and the cells contain starch, oil, or crystals. 
The special characteristics of the phelloderm are then described in a 
number of species. 
* Parenchyme-Sheath in the Leaves of Dicotyledons.; — Herr B. 
Schubert has examined the parenchymatous sheath which surrounds the 
vascular bundle in the leaves of all Dicotyledons examined with the 
exception of the Crassulaceae. He classifies them under two groups, 
according to their form and the amount of chlorophyll which they con- 
tain. In the first and much the most common type, the cells are 
elongated in the direction of the bundle, and are distinguished from the 
rest of the mesophyll by the absence of intercellular spaces. As a rule 
* Journ. Linn. Soc. (Bot.), xxxiii. (1897) pp. 118-22 (3 figs.). 
t Bot. Centralbl., lxxi. (1897) pp. 81-7, 113-21, 161-70, 193-200, 225-30 (1 pi.). 
+ Bot. Centralbl., lxxi. (1897) pp. 337-47, 385-95, 435-45, 465-76 ; lxxii. (1897) 
pp. 13-21, 61-9 (1 pi.). 
