ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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last case tliey differ from tlie corresponding colouring-matters of flowers 
and of fruits in being always derived directly from the protoplasm, and 
not from free-existing chlorophyllous leucites. 
(3) Structure of Tissues. 
Aerenchyme.^ — Dr. H. Schenck describes the structure of a tissue 
to which he gives this name, especially characteristic of the submerged 
portions of aquatic and marsh plants, and particularly of those which are 
woody, herbaceous species being frequently destitute of it. It springs 
from the phellogen, and is therefore homologous in its origin with cork ; 
it may be replaced by lenticels. Its cells are always thin-walled and 
not suberized, developing between them large intercellular spaces com- 
municating with one another and filled with air ; the cells themselves 
have an extremely thin parietal layer of protoplasm, and contain a small 
nucleus and minute leucoplasts, which sometimes develope into starch- 
grains, and a watery sap, but do not themselves contain air. In one type 
of structure these cells are elongated in a radial direction, and not 
arranged in regular zones ; in a second type they form concentric strata, 
each composed of a single layer of cells, and connected with one another 
by radial trabecules. The air inclosed in the intercellular spaces 
contains a smaller j)i*oportion of oxygen than that of the atmosphere. 
The author finds aerenchyme in species of Onagraceas, Lytbraceaa, 
Melastomaceas, Hypericaceac, Labiatae, Euphorbiaceae, Mimosese, and 
Papilionacere. Its function appears to be to facilitate the respiration of 
the parts of the plant in which it is found. 
Structure of Dicotyledonous Stems.t — Dr. E. Kaimann points out the 
existence of two types of structure in dicotyledonous stems. The first 
and simpler type occurs in most herbaceous and annual plants, and in 
a few woody stems, such as Aristolocliia, Clematis, and Atragene. The 
increase in thickness here proceeds entirely from the cambium, which, 
being formed between the xylem and phloem of the separate bundle- 
traces, gradually extends to the medullary rays, and thus becomes a 
closed thickening-ring, a portion of which in each new period of growth, 
as fascicular cambium, produces phloem and xylem, while a portion, as 
interfascicular cambium, gives rise to the elements of the medullary 
rays, so that the bundle-traces have a separate course even in older 
stems. In the majority of dicotyledonous woody plants we find, how- 
ever, a second and more complicated type. The leaf-trace-bundles do 
not here anastomose, as in the first type, but have blind endings in the 
stem ; and, furthermore, the structure of the leaf-traces is different in 
different parts of the stem ; while the leaf- traces of the upper leaves 
correspond in structure to the fascicular cambium, those of the lower 
leaves pass over into that of the interfascicular cambium. The inter- 
fascicular cambium is, therefore, not, as generally described, an ex- 
clusively cauline tissue. 
Periderm.J — M. H. Douliot has investigated the structure and origin 
of the periderm in plants belonging to a large number of natural orders, 
* Jalirb, f. Wiss. Bot. (Pringsheim), xx. (1889) pp. 526-74 (6 pis.). 
t SB. K. K. Zool.-Bot. Gesell. Wien, xxxix. (1889) pp. 52-6. 
X Anil. Sci. Nat. (Bot.), x. (1889) pp. 325-95 (64 tigs.). Cf, this Joiirnnl, 1889, 
p. 406. 
