352 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
with transverse septa. The secreting canals of the liber never abut on 
sieve-tubes. The sieve-tubes of Angiosperms are dispersed irregularly 
through the primary liber. There are two distinct types of sieve-tube, 
those of the gourd and of the vine ; but all intermediate forms may 
occur in the same plant. The elements separated from the sieve-tubes 
by tangential walls must be considered, like those separated by radial or 
oblique walls, as companion-cells. Besides the sieve-plates on their 
terminal walls, the sieve-tubes may have others, usually smaller, on their 
longitudinal walls. The liber-fibres may be united transversely by 
series of sieve-cells developed in the medullary rays. The formation of 
callus is due to excessive development of the thin layer of the membrane 
which covers the filaments of cellulose. The nucleus of the sieve-tubes 
usually disappears at an early period, but may sometimes be found in 
the parietal protoplasm of still active tubes. The contents of a sieve- 
tube in its active state consist of a thin layer of active parietal proto- 
plasm, and a large central vacuole containing water and albuminoid 
substances in solution. The companion-cells also contain abundance of 
albuminoids, but neither they nor the sieve-tubes contain starch. The 
duration of activity of the sieve-tubes varies greatly, as also does the 
period of the appearance of the callus. Seedlings of the gourd kept in 
the dark developed abundant callus in the liber of the hypocotyl 
(tigellum) ; while those exposed to full light had their sieve-plates 
perforated. 
Collenchymatous Cork.* — Herr H. Molisch describes a tissue of a 
peculiar character which he finds immediately beneath the epiderm of 
the fruit in several varieties of Capsicum. It partakes of the very 
different characters of cork and collenchyme, resembling ordinary 
parenchymatous collenchyme in its appearance, contents, and mode of 
thickening, but presents none of the characteristic reactions of cellulose, 
being, on the other hand, strongly suberized. It is two or three layers 
of cells in thickness, and of a golden-yellow colour. 
Thyllae.t — Herr H. Conwentz describes the structure and mode of 
formation of thyllae or similar structures, especially in the wood of those 
fossil trees which produce amber, where they occur exclusively in the 
root. Their structure here is the same as that in existing plants, where 
they are sometimes characteristic of entire natural orders, sometimes of 
particular genera. They result from the growth of the closing mem- 
brane of the bordered pits which lie on the common wall of a tracheid 
or vessel and of a parenchym e-cell. Similar structures are found also 
in resin-canals, as the results of the growth of the epithele-cells into the 
intercellular spaces. They may also be the result of injury. A thylla 
has always the same physiological purpose, viz. to cut off an organ 
which is no longer performing its function. 
Secondary Vascular Bundles of the Arborescent Liliaceae.t— 
Dr. P. Roseler gives further arguments in favour of his contention 
that in the arborescent Liliaceas every rudiment of a vascular bundle 
* Ber. Deutsch. Bot. Gesell., vii. (1889) pp. 364-6. 
t T. c., Gen.-Versamml.-Heft, pp, 34-40. Cf. this Journal, 1888, p, 988. 
X Bot. Ztg., xlviii. (1890) pp. 26-30. Cf. this Journal, 1889, p, 657. 
