.514 ON THE ORGANS AND MODE OF FECUNDATION 



713] the tissue of stigma, in the cells of the ovulura even 

 before impregnation, and in all the stages of formation of 

 the grains of pollen, the evolution of Avhich 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 TanlcervillicB, has more particularly noticed 

 it, and seems to consider it as only visible after impregna- 

 tion. 



7H] The second point of structure in Orchidese to which 

 I shall at present more briefly advert, is the frequent exist- 



single series of particles. The course of these currents seems ofteu in 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 Tradeicantla virgimca, while 

 the antherse are yet colourless, their loculi are filled with minute lenticular 

 grains, having a transpareut 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 either 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 where one transparent liue 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 slage 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 yellow colour, and is now the perfect grain of pollen. 



