ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
207 
tissues through which it forces its way. These pores appear, however, 
to have only a temporary existence, closing up again after the granules 
have forced their way through. In some cases a very large terminal 
opening is formed, which may be from half to one-third the diameter of 
the tube. Both direct sunlight and total darkness are unfavourable to 
the growth of pollen-tubes, a feeble light being the most advantageous ; 
the optimum temperature is between 20° and 30° C. Both branching 
and septation are not uncommon phenomena in pollen-tubes ; the septa 
are the result of ingrowths of the wall of the tube. The branching may 
be either dichotomous or apical. Pollen-grains of Mercurialis may emit 
two or even three separate tubes. 
Polyembryony.* * * § — Dr. K. Schilbersky distinguishes two principal 
kinds of polyembryony — one resulting from the ovum-cell, the other 
adventitious ; the embryos resulting from the former he terms true, from 
the latter pseud-embryos ; the latter are simply a modification of non- 
sexual propagation within the ovule. Polyembryony occurs normally 
in some families, as in the Conifer®, at least in the early stages of the 
development of the embryo. The author regards the embryonic vessel, 
together with the synergids in Angiosperms, as analogous to the 
numerous central cells in the embryo-sac of Conifer®. 
Mr. E. 0. Jeffrey! describes the frequent occurrence of several 
embryos in the embryo-sac of Erythronium americanum. In some cases 
the number of embryos is very large, but there is never more than one 
in the ripe seed. The phenomenon appears to be exactly homologous 
with the polyembryony of the Gymnosperms. 
Cross-Pollination and Self-Pollination. — Dr. P. Knuth J states that 
both the short-styled and long-styled flowers of Primula acaulis are 
abundantly visited by Bombus hortorum , which also visits neighbouring 
flowers of P. officinalis and P. elatior , so that frequent hybridisation 
may take place. All three species are also visited less frequently by 
Lepidoptera and Bombyliid®. 
The same writer § deduces the following laws from the facts already 
accumulated with regard to the visits of insects to plants : — The more 
complicated the structure of a flower, and the deeper its honey is con- 
cealed, the less are the insect-visitors dependent on the insect-flora of 
the district, and the more do they belong to the same or to similar 
kinds ; while the nearer the honey lies to the surface, the more do the 
insect- visitors vary in different regions, and the more is fertilisation 
dependent on the species characteristic of, or peculiar to, the district 
in question. 
From experiments made, chiefly on the dahlia, by covering up the 
outer flowers of the capitule with coloured papers, Prof. F. Plateau || 
comes to the conclusion that insects are not attracted visually by the 
bright colour of the flowers, but by some other sense, probably that of 
smell. 
* In Hungarian. See Bot. Ceutralbl., lxiv. (1895) p. 229. 
f Ann. Bot., ix. (1895) pp. 537-41 (1 pi.). 
i Bot. Centralbl., Ixiii. (i895) pp. 97-8. Cf. this Journal, 1893, p. 66. 
§ ‘Die I lutenbesuoher derselbcn Pflanzen-Art in verschiedener Gegenden,’ Kiel, 
1895. See Bot. Centralbl., xliv. (1895) p. 83. 
|| Bull. Acad. R. Sci. Belg., xxx. (1895) pp. 466-87 (1 pi.). 
