152 
ORGANOGRAPHY. 
BOOK I. 
remains of which may occasionally be found among the grains 
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 anther£e 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 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 the 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 
