152 ORGANOGRAPHY. BOOK I. 



remains of which may occasionally be found among the gi-ains 

 of pollen. 



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 anthers are yet colourless, their loculi 

 are filled with minute lenticular gi'ains, having a transparent 

 flat limb, with a slightly convex and minutely granular semi- 

 opake disk. This disk is the nucleus of the cell, which pro- 

 bably 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 di- 

 visions 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 con- 

 sisted 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 in- 

 crease in size, and in tVie thickness and opacity of its mem- 

 brane, acquires a pale yellow colour, and is now the perfect 

 grain of pollen." {On Orchid, and Asclep. p. 21.) 



There are no observations, however, upon this subject which 

 can be compared to those of Mirbel for clearness of descrip- 

 tion, elaborate detail, and beautiful illustration. By beginning 

 his enquiry at the very earliest period, when the organisation 

 of the anther can be discovered, he has been enabled to ex- 

 plain what was before obscure, and to correct what has been 

 either inaccurately or imperfectly described. In 1832, he 

 examined the developement of pollen in the anther of a 

 Gourd. He states that " when the flower bud of this plant 

 is about a line in length, each lobe of the anther is entirely 

 composed of cellular tissue, the bladders of which present in 



