182 
ORGANOGRAPHY. 
BOOK I. 
To this important part of the organisation of perfect 
plants, attention has been directed with great care and skill 
by numerous observers of the first class, among whom are 
especially to be noticed R. Brown, Ad. Brongniart, Fritzsche, 
Griffith, Mirbel, Mohl, and Schleiden. From their enquiries, 
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 gra- 
nular 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 antherae^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 increase in size, and in the thickness and opacity of its 
