Fecundation in Orchidea and Asclepiadea. 713 



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 1 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 so little 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 TankervilUa, 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. 



* In the very early stage of the flower bud of Tradescantia virginica, vi^hile the an- 

 theras 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 distinguisliable, 

 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 imcleus. 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. 



4 Y 2 The 



