Fecundation in Orchidee and Asclepiadee. 113 
in the cells of the ovulum even before impregnation, and in all 
the stages of formation of the grains of pollen, the evolution of 
which is so remarkable in those species of Tradescantia*. 
The few indications of the presence of this nucleus, or areola, 
that I have hitherto met with in the publications of botanists, 
are chiefly in some figures of epidermis, in the recent works of 
Meyen and Purkinje, and in one case in M. Adolphe Brongni- 
art's memoir on the structure of leaves. But solittle importance 
seems to be attached to it, that the appearance is not always 
referred to in the explanations of the figures in which it is 
represented. Mr. Bauer however, who has also figured it in the 
utriculi of the stigma of Bletia Tankervillie, has more particu- 
larly noticed it, and seems to consider it as only visible after 
impregnation. 
proof that this body does not adhere to both sides of the cavity, and also that the 
number and various directions of the currents cannot be owing to partial obstructions 
arising from the unequal compression of the cell. 
* [n the very early stage of the flower bud of Tradescantia virginica, while the an- 
there are yet colourless, their loculi are filled with minute lenticular grains, having a 
transparent flat limb, with a slightly convex and minutely granular semi-opake disk. 
This disk is the nucleus of the cell, which probably loses its membrane or limb, and, 
gradually enlarging, forms in the next stage a grain also lenticular, and which is marked 
either with only one transparent line dividing it into two equal parts, or with two lines 
crossing at right angles, and dividing it into four equal parts. In each of the quadrants 
a small nucleus is visible; and even where one transparent line only is distinguishable, 
two nuclei may frequently be found in each semicircular division. These nuclei may 
be readily extracted from the containing grain by pressure, and after separation retain 
their original form. | 
In the next stage examined, the greater number of grains consisted of the semicir- 
cular divisions already noticed, which had naturally separated, and now contained only 
one nucleus which had greatly increased in size, ——— 
In the succeeding state the grain apparently consisted of the nucleus of the former 
stage considerably enlarged, having a regular oval ‘form, a somewhat granular surface, 
and originally a small nucleus. This oval grain continuing to increase in size, and in 
the thickness and opacity of its membrane, acquires a pale yellow colour, and is now 
the perfect grain of pollen. 
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