386 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the third of these. The cotyledons and horizontal leaves have stomates 
on the under side only, the vertical leaves on both sides. The glands, 
which are apparently a protection against excessive radiation, are dis- 
tributed irregularly in the leaves, as also in the leaf-stalk, receptacle, 
and walls of the ovary ; they are of lysigenous origin, resulting from the 
complete absorption of a secreting system. The vascular bundles of 
the stem are bicollateral ; in the leaves the bundles are bicollateral 
in the stronger veins, passing into collateral in the finer veins. The 
author regards the horizontal as the original form of leaf, the vertical 
form being an adaptation protecting against the excessive transpiration 
resulting from too strong an exposure to light. 
Leaves of Palms.* — From an examination of species belonging to 
52 out of the 128 known genera of palms, Herr K. Zawada states that 
members of the same tribe, and even of the same genus, are distinguished 
by common anatomical characters. In the first rank are characters 
derived from the nature of the upper, and, when this is wanting, of the 
lower mid-rib, and from the presence or absence, and the nature of the 
hypoderm ; and, subsidiary to these, the presence or absence and the 
nature of the stomates, and of the ring of sclereuchyme ; the nature of 
the epiderm and mesophyll ; the presence or absence of trichomes, 
raphides, and tannin-sacs ; the presence of one or more phloems and of 
pitted vessels in the large vascular bundles ; and the arrangement of 
the veins in the lamina. 
0. Physiology. 
(11 Reproduction and Embryology. 
Phenomena of Impregnation. - ! — M. L. Guignard describes in detail 
the researches hitherto made by others in the morphological phenomena 
of impregnation, together with his own most recent observations, and 
compares the results with those obtained in the animal kingdom by Van 
Beneden, Fol, Hertwig, and others. 
In the formation of pollen-grains he distinguishes between the pri- 
mordial mother-cells and the definite mother-cells of the pollen, the 
former being an earlier condition of the latter. It is at the moment when 
the cells of the tapetal layer are differentiated that cell-division ceases 
in the primordial mother-cells, and they become definite mother-cells. 
Before arriving at maturity the pollen-grain of Angiosperms divides 
into two cells of unequal size, the smaller of which is the generative, 
the larger the vegetative cell ; they differ from one another in form, 
structure, and reactions. The generative cell becomes free in the pollen- 
grain at a more or less advanced period of development, and usually takes 
the form of a lens or crescent having the nucleus in its centre. The 
generative nucleus, together with the protoplasm which surrounds it, 
undergoes a bipartition which (in Liliurn Martagon) takes place only in 
the pollen-tube. It is sometimes the generative, sometimes the vege- 
tative nucleus, which first enters the tube. In its course through the 
* ‘ Das anat. Verhalten d. Palmenblatter,’ Karlsruhe, 1890, 40 pp. See Bot. 
Centralbl., 1891, Beih., p. 517. 
t Ann. Sci. Nat. (Bot.), xiv. (1891) pp. 162-288 (10 pis.). Cf. this Journal, ante, 
p. 64. 
