352 ORIGIN OF POLLEN. [BOOK i. 



we have arrived at a knowledge of the history of the pollen, 

 notwithstanding its minuteness, from its first secretion to its 

 final destruction, after the important purpose for which it is 

 provided has been attained. 



The origin of the pollen, according to the testimony of all 

 observers, occurs in the cells of which the anther is composed, 

 and appears to consist in a peculiar organisation of their 

 granular interior. The grains are usually produced in fours, 

 by the bisection of their generating cell in two opposite 

 directions, but are occasionally formed in pairs or singly. 



In 1831, Brown speaks thus of the evolution of the pollen 

 of Tradescantia virginica. " In the very early stage of the 

 flower bud, while the antherse are yet colourless, their loculi 

 are filled with minute 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 

 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 often 

 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 some- 

 what 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." (On Orchid, and 

 Asclep, p. 21.) 



In 1832, Mirbel examined the development of pollen in 

 the anther of a Gourd. He states that " when the flower-bud 



