ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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forming long tubes. The whole appears to be a contrivance for the 
promotion of aeration. 
C4) Structure of Organs. 
Resemblances in Habit between Plants belonging to different 
Genera.* — Dr. C. Reiche points out the frequent occurrence of a close 
resemblance in external appearance between (1) two species belonging to 
different genera of the same order, (2) two species belonging to widely 
separated genera. This resemblance must be largely due to the influence 
of external conditions, and is not an example of mimicry in the true sense 
of the word ; and the author suggests the need of caution in explaining 
similar resemblances in the animal kingdom as necessarily the result of 
mimicry. 
Structure of Pollen.t — Dr. P. Biourge has investigated the structure 
of pollen-grains obtained from a large number of plants — Dicotyledons, 
Monocotyledons, and Gymnosperms — especially in relation to their 
chemical constitution. He finds that pollen-grains always have two 
coats, an extine and an intine. Among Dicotyledons they present two 
general types, — (1) Spherical, with numerous pores and no furrows ; 
(2) ellipsoidal, with three pores ; the ellipsoid may be flattened or 
elongated, and the pores vary from the rounded to the elongated form, 
or may pass into furrows, often reaching the poles. In Monocotyledons 
simple grains have usually only one furrow ; the primitive form is that 
of a quarter of an orange. In both groups compound grains occur. The 
extine is composed of one or two layers, the outer of which is usually 
sculptured ; the number of pores and furrows varies, sometimes even in 
the same species. The intine is always continuous ; if there are pores, 
it is composed of several layers, of which one is always closed ; between 
the pores there are frequently thickenings. The wall of the pollen-tube 
is always formed by a drawing out of the intine ; it may be simple or 
composed of several layers. 
The extine is rarely composed entirely of cellulose, and is usually 
cutinized ; while the intine is composed of pure cellulose, or of pectic 
substances, or of a mixture of the two ; there are often special layers of 
callose. The wall of the pollen-tube is generally composed of cellulose. 
The wall of the pollen-mother-cells is thickened by the apposition of 
secondary layers before the division into tetrads ; and the same takes place 
in each daughter-cell. The extine appears before the intine ; the inner- 
most layer of the latter frequently only a short time before dehiscence. 
Staminal Hairs of Thesium.J — Miss M. F. Ewart describes the 
hairs which are found attached to the perianth-tube behind the stamens 
in various species of Thesium and in other allied genera of Santalaceae. 
These hairs are of two kinds — those which are comparatively short and 
thick, and directed downwards towards the base of the style, and those 
which are long and slender, and directed upwards towards the top of 
the anther. The hairs are unicellular, and contain a yellowish-green 
semi-fluid secretion which gives the microchemical reactions of a balsam ; 
they have also a basal cushion and a small rounded terminal cap. They 
* Yerhandl. Deutsch. Wiss. Yer. Santiago, ii. (1892) pp. 243-5 (1 pi.), 
f La Cellule, viii. (1892) pp. 45-76 (2 pis.), 
t Ann. Bot., vi. (1892) pp. 271-90 (1 pi.). 
