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 514 ON THE ORGANS AND MODE OE FECUNDATION

/13- the tissue of stigma, in the cells of the oviiluni 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 Brongniart's memoir on the structure of leaves. But so little importance seems to be attached to it, that the appearance is not always referred to in the ex- planations of the figures in which it is represented. Mr. Bauer, however, who has also figured it in the utriculi of the stigma of Bletia Ta/ikervillice, has more particularly noticed it, and seems to consider it as only visible after impregna- tion.

7Uj The second point of structure in Orchideae to which I shall at present more briefly advert, is the frequent exist-

single series of particles. The course of these currents seems ofleu iu some degree affected by the nucleus, towards or from which many of them occa- sionally tend or appear to proceed. They can hardly, however, be said to be impeded by the nucleus, for they are occasionally observed passing between its surface and that of the cell ; a 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 com- pression of the cell.

^ In the very early stage of the flower -bud of Traclescantia virgiifica, while the autherae are yet colourless, their loculi are tilled with minule lenticular grains, having a transparent flat limb, with a slightly convex and minutely granular semi-opaque disk. This disk is the nucleus of the cell, which proba- bly loses its membrane or limb, and, gradually enlarging, forms in the next stage a grain also lenticular, and which is marked eitlier with only one trans- parent 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 M-hcre one transparent line only is distinguish- able, 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 semicircular 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 jellow colour, and is now the perfect grain of pollen.

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